forged piston question

[acannell]

So heres my situation:

2.5 common block, bored for 0.030" over pistons.

All 4 cylinders are within factory spec for taper and out of round.

#2, #3, and #4 measure less than 0.001" out of round.

#2, #3, and #4 measure less than 0.001" taper.

#2, #3, and #4 all measure between 3.474" and 3.475".

#1 is the problem child.

#1 measures 0.0015" out of round at the top, and 0.0019" taper.

This is because I did alot of honing to remove a scratch. Glad I did, otherwise option was to scrap the block and do alot of machining. Worked out fine and had good compression.

If it werent for that out of round, #1 would fit in with the other bores as far as being between 3.474" and 3.475". Most of the bore does. But just that one measurement at the top perpendicular to the crank measures 0.0015" greater than the nominal of all the other bores.

So my question is, if I size the forged pistons for the #2, #3, and #4 bores, will it still be within spec of the #1 bore?

I suppose worst case I could size them on the small side and have more piston slap. And then the fitment question would come down to ring gap. And in that case, if I file fit the rings, then there is no issue with #1 being bigger?

Assuming that the OOR and taper are not problems in themselves. They are within factory spec and I'm willing to go with them as-is. Worst case I have to scrap the block which is what I would need to do anyway. Then a new block would get bored to match the forged pistons. So there is no risk here in trying it.

Since its within spec for the factory and the rings would be the same anyway, then is it actually totally irrelevant that I'd be switching to forged, if they are sized for the same goal-bore as the stock cast pistons?

[onerippinturbo2]

you wouldnt need to scrap the block, there is the .040 oversized option too.

[bamman]

With those numbers, commonly available pistons available from Turbos Unleashed or FWD Performance, and file fit piston rings, you should have no problem using that block.

Everything is in spec.

If you are still paranoid about the numbers, you can have your block machined .040 over. Give the machine shop your block, pistons, the piston kit specifications supplied with your pistons, and the FSM specifications and tell them to bore and hone it. You can file fit the piston rings yourself.

[acannell]

With those numbers, commonly available pistons available from Turbos Unleashed or FWD Performance, and file fit piston rings, you should have no problem using that block.

Everything is in spec.

If you are still paranoid about the numbers, you can have your block machined .040 over. Give the machine shop your block, pistons, the piston kit specifications supplied with your pistons, and the FSM specifications and tell them to bore and hone it. You can file fit the piston rings yourself.

As far as I can tell, FWDP offers only 0.912 pins which I want to avoid because that means machining the rods and paying for new pins.

TU offers 0.901 pin pistons but I'm not sure if they will sell just the pistons. The $565 shipped price they list is pistons + rings + locks + pins.

Wiseco says $530 shipped for pistons + rings + locks. So I think TU's price is pretty good.

I just need to make sure that whatever I get is matched to the bores properly. So I need to know what bore size what TU sells is designed for and how that will interact with my bores.

If I order the pistons to have the right wall clearance at 3.474", then they will have 1 thou more clearance on some of the cylinders at some points, and 2.5 thou more clearance at the top of cyl #1. Id just like to hear from the manufacturer what they think about that. I guess what that boils down to is how tolerant is the forged piston to added wall clearance above its nominal value.

[Ondonti]

When you get the pistons and you are in full picky mode as you have noted above, you would measure each piston and match them to the best fitting bore. This would include further honing if you didn't like how things matched up.

Ring gaps will be done custom for each cylinder so its more about the diameter of the rings when you have a "bad" cylinder. Pretty sure I have been on the wrong end of a not perfect ring diameter issue with a motor of mine. I blame some of that on Venolia's giant clearance requirements when they did not also sell matching rings with the piston so I had to source them elsewhere.

Buy pins with pistons if thats how they are sold. Don't cheap out on 30 bucks. Stock pin size is not as strong so use what comes with the pistons so you can blame somebody besides yourself if there is a problem with the pins bending.

I don't really understand how you were planning on custom ordering a piston for a "tapered out of round bore" anyways. Stick to the actual bore size, and I would say most importantly, whatever the smallest measurement is is where you will run into problems. If it was out of round BUT also the smallest bore size, you would have to be careful. Order the right size pistons and if one happens to be slightly smaller, put in in the smallest bore. Best would be owning your pistons already before final honing and then honing the block to perfectly match each pistons :P Hard to do when you have a damaged cylinder. I have a block I want to reuse but its hard to tell if the damage in it would be removed with the planned 0.100" overbore.

[acannell]

When you get the pistons and you are in full picky mode as you have noted above, you would measure each piston and match them to the best fitting bore. This would include further honing if you didn't like how things matched up.

Ring gaps will be done custom for each cylinder so its more about the diameter of the rings when you have a "bad" cylinder. Pretty sure I have been on the wrong end of a not perfect ring diameter issue with a motor of mine. I blame some of that on Venolia's giant clearance requirements when they did not also sell matching rings with the piston so I had to source them elsewhere.

Buy pins with pistons if thats how they are sold. Don't cheap out on 30 bucks. Stock pin size is not as strong so use what comes with the pistons so you can blame somebody besides yourself if there is a problem with the pins bending.

I don't really understand how you were planning on custom ordering a piston for a "tapered out of round bore" anyways. Stick to the actual bore size, and I would say most importantly, whatever the smallest measurement is is where you will run into problems. If it was out of round BUT also the smallest bore size, you would have to be careful. Order the right size pistons and if one happens to be slightly smaller, put in in the smallest bore. Best would be owning your pistons already before final honing and then honing the block to perfectly match each pistons :P Hard to do when you have a damaged cylinder. I have a block I want to reuse but its hard to tell if the damage in it would be removed with the planned 0.100" overbore.

The pistons are sold how you order them. Just because TU or FWDP sells them in a kit with pins and rings doesn't mean that have to be ordered that way for some reason. Its not just the cost per pin, its the cost per pin + having the rods remachined + the inevitable screw-ups and delays dealing with a third party machine shop + the zero benefit for all of that besides a slightly stronger pin that I don't need and can't justify.

All bores are tapered and out-of-round, including freshly machined ones, its just a matter of how much. Im not doing anything special by ordering the pistons made to a certain size. My bores are in-spec but one is at the edge and I want to make sure that the piston diameter I order fits in the tolerance band for all 4 including the 1 that has more variance. Wiseco and Venoila both told me that 100% of the pistons they make for our cars are machined on demand from scratch, none are off the shelf. So if FWDP or TU are stocking up, its because the idea is to have people machine the block to the piston, which is not what I'm doing.

There is no single "actual bore size". All 4 bores are different from each other, like on any block, even a freshly machined one, they also differ from the standard 0.030" oversize. So the goal here is to pick a single piston size that will work with all 4. If one bore was bigger than the others by 0.005" then it would be out of the question. But all 4 are in spec so I think its reasonable to do.

[bamman]

So I need to know what bore size what TU sells is designed for and how that will interact with my bores.


Maybe this will help, from my TU .020" over pistons:

http://i16.photobucket.com/albums/b49/omni16531/Static/pistonInfo_zpsa4155e27.png

[acannell]

Maybe this will help, from my TU .020" over pistons:

http://i16.photobucket.com/albums/b49/omni16531/Static/pistonInfo_zpsa4155e27.png

Very interesting thank you!

Did you have your bores machined to match your pistons or the other way around?

[Warren Stramer]

Asa, Why not have the block plate honed to 3.477,(or whatever it takes to clean up the bores to the same dimension and concentricity) getting rid of the out-of-round, and taper. You then you have four perfectly matching bores. Then, Call wiseco and tell them to make four pistons to fit your bore diam. minus skirt clearance. I've had them make pistons to any dimension and pin size, and the cost is just a little more.

[acannell]

Asa, Why not have the block plate honed to 3.477,(or whatever it takes to clean up the bores to the same dimension and concentricity) getting rid of the out-of-round, and taper. You then you have four perfectly matching bores. Then, Call wiseco and tell them to make four pistons to fit your bore diam. minus skirt clearance. I've had them make pistons to any dimension and pin size, and the cost is just a little more.

Time and money. Dealing with machine shops is pretty much the worst experience I've had the entire 7 years I've had the car. The delays, the screw ups, the costs. I would not at all be surprised if I had a machine shop "fix" the cylinders only to have them measure even worse then they currently do. I guess alot of people on these forums have great machine shop stories but I certainly dont, I will not go to one unless I have absolutely no choice. And I think technically my mill has enough Z travel to bore the block here but I dont even want to take the block out or deal with any of that. I just want to wave my teeny magic wand and change the cast pistons to forged and get back to what I was doing, which was prepping the daytona to be a test platform for the BMFi and other products like swingvalves or who knows what. If I can get it to hold together long enough to do that its mission accomplished.

Plus I dont like doing work that I cant justify. If the bores are in-spec, then why do they need to be fixed? Even if I had them cleaned up they'd end up wearing to where they are now anyways, right? They were last bored around 30k miles ago. I dont like the idea of always trying to maintain the bores to a certain specification that is arbitrarily tighter than factory without a very good reason.

Below are the measurements. Besides the one anomaly at the top of #1 where I went crazy with the harbor freight hone to get rid a of a scratch, all measurements are within 0.0011" of 3.474".

http://i242.photobucket.com/albums/ff197/acannell/boremeasurements_zpsa68458c9.jpg
http://www.shanalogic.com/media/catalog/product/cache/1/image/9df78eab33525d08d6e5fb8d27136e95/9/1/9185_sha_wand_full2.jpg

[thedon809]

I hear you about the lack of good machine shops. I have 2 around me. One is known for screwing people and the other is an old ornery drunk.

[acannell]

Just spoke to Mike @ FM

He says that in my situation hed recommend getting file fit rings and going with around 0.015" ring gap for the 450hp power level.

Last time I measured they were .018 to .021 gap.

He says pistons + 0.912 pins + locks are $540 and rings are an extra $85.

He says that there are no good pins in 0.901 and that he recommends going to 0.912 even if it means the extra work to hone the rods bigger. Which is reasonable because its really only maybe $10 each.

But I dont want to do that simply because I dont want to deal with a machine shop for any reason.

But anyhoo theres the data from JE.

- - - Updated - - -


I hear you about the lack of good machine shops. I have 2 around me. One is known for screwing people and the other is an old ornery drunk.

I hear ya!! Grumpy jerks I dont want them having anything to do with my project.

[Vigo]

So if there are no 'good' .901 pins, what makes the available ones 'bad'?

[acannell]

So if there are no 'good' .901 pins, what makes the available ones 'bad'?

Im not saying I agree with him. I think thats BS personally. Just throw it on the pile of other made up crap that you have to wade through in automotive land. If it cant be proven and should be done "while in your in there" because its "just a little bit more" and "wouldnt it be a shame" if it broke because you didnt, then superstition and anxiety takes over and fear gets turned into fact.

[bamman]

Very interesting thank you!

Did you have your bores machined to match your pistons or the other way around?

Everything was centered around the pistons that I brought them. The machine shop wouldn't touch my block without the pistons and that sheet of paper posted above. To this day that engine runs like a champ and is so quiet.

Going back to your block, everything is within spec. I would buy some Wiseco pistons from TU (they have the stock wrist pin size and they are readily available) and file fit the rings yourself and call it a day.

As far as ring gaps, I used the piston manufacturer specs, which also was within the FSM specs, and filed the rings myself.

I remember before making the switch to forged pistons I was researching everything to do, from recommended ring gaps, to using a hone plate, to line boring and was so pressured to "doing it right." But at the end of the day I did everything to specs in the FSM and I can sleep at night. The engine runs good, I like it, and life is good. Best of all, besides the boring and honing, I did it myself.

[Ondonti]

So if there are no 'good' .901 pins, what makes the available ones 'bad'?

I feel like advice is just being blown off here looking for the easy way out. If you are being picky about things being perfect, there is no easy way out.

There is a thread from earlier this year with Cindy talking about the documented weakness of the stock size pin. Pin size plus the support provided by the bosses on the piston pretty much lock you into a torque/cylinder limit. Selling larger/better pins is not a gimmick. This goes across all engine platforms. Now, you only need upgraded pins if you will make enough HP to justify.

I also don't like those ring gaps, especially since Acannell likes pump gas. In FM's glory days, everybody was running race gas and still barely holding together motors or doing rebuilds in the pits.

Its a fact that the pistons determine bore size during proper machining. If your block is already completed then you can't do any better than shoot dice and see how they come out and how they match the block. If you don't want to bore farther than you are stuck. Nobody is going to modify a piston for a slightly out of round or tapered cylinder. That is not how it works. If you want things perfect, then the only option is to follow Warren's advice. I would still have the pistons ordered and then the bores can exactly match each piston's real life size.

[Pat]

^^^ agree. If you're looking for perfect, you have to machine. Can it work with a slightly out of spec cylinder? Yes. Will it be perfect for longevity or power? No.


Also agree on the ring gaps. .015 is a tight gap and if I remember correctly, right in the middle of the stock spec. I've done my motors much looser to add a margin of safety in case I get something bit hotter than expected and have not experienced any issues with oil consumption or blow by. Motor in my drag week car was set up at .019 top, .021 second. Motor in my grm car was at .022/.024 when I put it together in 2010.

[knownenemy]

I'm ALL FOR trying new ways to avoid troubles in building engines.
But, the current, proven methods are just fine.

You know what they say, "If it ain't broke, don't fix it".

I've dealt with Mike DeMoss for many years, and every bit of advise I've ever got from him was spot on.
When I built GLHS #0309, Mike was the man who kind of led my decisions.
2.2 with JE's, rings file fitted to .015, and .020, 20-25lbs boost; no issues.
Engine ran like a RAPED APE!!!
At one point, the WG was stuck closed, and I was over boosting regularly to 30+ psi; and I still never had an issue.
(other than the a525 exploding so hard a piece of the case hit the inside of the hood so hard it made a dent from the inside!) :)

Asa, in my experience machine shops are run by old knowledgeable men, who've seen it all.
They are usually rough on the phone, but there's a solution to their somewhat ignorant attitude; Money.
When you arrive at their shop with your block and the pistons you choose, with a handful of cash,
they'll be some of the nicest, most respectful and helpful people you'll meet. :thumb:

[Ondonti]

^^^ agree. If you're looking for perfect, you have to machine. Can it work with a slightly out of spec cylinder? Yes. Will it be perfect for longevity or power? No.


Also agree on the ring gaps. .015 is a tight gap and if I remember correctly, right in the middle of the stock spec. I've done my motors much looser to add a margin of safety in case I get something bit hotter than expected and have not experienced any issues with oil consumption or blow by. Motor in my drag week car was set up at .019 top, .021 second. Motor in my grm car was at .022/.024 when I put it together in 2010.

I also forgot to mention that another reason its good to have your pistons before final machining is that each manufacturer has their own callouts on piston to cylinder clearances and there are different alloys (expansion rate changes). Seems like a good way to save a little money and risk wasting a lot.

I am a staunch advocate of loose ring gaps in turbo engines. No point increasing how perfectly all your systems have to function to get away with tight gaps when there is zero performance gain. I think there are about 10 threads in the past few months showing how Acannell's car has had some problems just like the rest of ours do :(

When really nailing down perfect ring gap, that is also going to depend on your fuel choice, the location of your ringlands (how hot they will get) etc. I can't determine a ring gap based purely on a PSI number. Ring gap is something you figure out by trial and error, or you go big enough based on other peoples problems to avoid error.
I really can't fathom the idea of not filing your rings to fit though, I can't imagine a serious motor was ever built by a true professional with out of the box ring gaps.

I was running .022 .026 in my daily and had to bump up to .024 and .028. The .022 broke piston towing, the .024 pushed coolant. I know which item I prefer to be my fuse.

Someone who runs near 30psi boost with a mild upgrade turbo and pump gas really needs an extra margin of safety. When we are talking 2.2/2.5 ring gaps (with different strokes and rod ratios), even the advice people give not only depends on the backpressure in your turbo system, the inefficiency of your intake system, but the cylinder head style you chose (huge difference in knock) , the port job, the cams, and how well the piston matches the combustion chamber....oh, coolant temp, oil temp, octane, accuracy of ignition timing blah blah blah. Safe for one person does not make it safe for you and vice versa. The idea that big ring gaps are bad is about the only "bunk" being talked about here. The only real downside is your oil getting dirtier sooner and the actual difference is tiny with ring gaps people actually use. I have seen a comparison of a small OEM style gap vs some huge .080 ring gap and the oil change interval went from 4k miles to 3k miles "suggested."

BTW I looked for the article months ago and still can't find it but here is someone's memory of the statement about gaps 3 years ago.

"I recently saw a test where 0.015" ring gaps where compared to 0.085" ring gaps. And oil usage went up from 1 quart /4000 miles to 1 quart/3500 miles for the bigger gap. Not a big difference in oil usage."

My memory of the article was that it was written by a major manufacturer, and not based off some engine builders one time back to back dyno on different engines which we should all know will make different amounts of power for various reasons, not just ring gap.

[Vigo]

There is a thread from earlier this year with Cindy talking about the documented weakness of the stock size pin.

I looked for a couple minutes but couldnt find this. Got a link or some helpful search terms?

[thedon809]

Would the pin being .011 bigger really make that big of a difference? That would mean you are also removing material from the small end of the rod potentially making it weaker, no?

[Ondonti]

I looked for a couple minutes but couldnt find this. Got a link or some helpful search terms?

http://www.turbo-mopar.com/forums/showthread.php?13500-The-Big-Rod-Discussion!&p=1046789&viewfull=1#post1046789


That's a valid point! I haven't seen any issues with the .912 pins under 400hp, but I know several guys in the 400+ range that were trashing the pins and needed to move up on the pins to a thicker wall version. I know one guy was using the srt .866 piston pins and his were in a bad shape with only one season of racing.
We do sell the upgraded pins for people that plan to make mega hp, but they aren't cheap!

Of course pins come in different alloys, qualities, thickness, and the design of the piston can make the pin endure less stress, or more stress. A pin boss that forces the pin to do all the work (imagine a long bridge vs a short bridge) would make a huge difference but stronger bosses probably add weight or force you to run even less material in other places to keep weight down.

[acannell]

I feel like advice is just being blown off here looking for the easy way out. If you are being picky about things being perfect, there is no easy way out.

There is a thread from earlier this year with Cindy talking about the documented weakness of the stock size pin. Pin size plus the support provided by the bosses on the piston pretty much lock you into a torque/cylinder limit. Selling larger/better pins is not a gimmick. This goes across all engine platforms. Now, you only need upgraded pins if you will make enough HP to justify.

I also don't like those ring gaps, especially since Acannell likes pump gas. In FM's glory days, everybody was running race gas and still barely holding together motors or doing rebuilds in the pits.

Its a fact that the pistons determine bore size during proper machining. If your block is already completed then you can't do any better than shoot dice and see how they come out and how they match the block. If you don't want to bore farther than you are stuck. Nobody is going to modify a piston for a slightly out of round or tapered cylinder. That is not how it works. If you want things perfect, then the only option is to follow Warren's advice. I would still have the pistons ordered and then the bores can exactly match each piston's real life size.

Im not saying its a gimmick. I'm saying there is no way to separate someone making a reasonable guess about it ("bigger is better") to it being proven to being a necessary upgrade. There is a difference.

A .912 pin, has at most 5% more stiffness than a .901 pin and a negligible increase in strength unless there is a change in material. How can that possibly make any difference? Maybe the .912 wall thickness is actually thinner than a .901 and it ends up being weaker, who knows? What exactly was the failure and what were the test circumstances? What were the materials and geometry of the pins being tested? What sort of stresses were the pins subjected to?

The idea you can have perfectly circular bores is a fantasy. The bores change shape when you replace a honing plate with a head, or start the engine and add coolant pressure, and uneven heating coming in pulses dissipating through an object with all kinds of varying wall thicknesses and heat transfer mechanisms. And they change shape again when you add a few thousand miles of wear. And when power levels go up.

http://www.aera.org/engine-professional/cylinder-bore-distortion/

There is no fact that pistons determine bore size. I can have pistons made all day long to match whatever bore diameter I want. In fact thats exactly what I'm going to do in the next couple days. Im not asking for a piston to be modified especially for an out of round or tapered condition. And by the way, you certainly could have them modify it that way if you wanted, they will make anything you want. Although Im not sure that even makes sense since the rings are what deal with that unless you make the piston so big it seizes. Im just making sure that the bore diameter I tell them to make the pistons for ends up being a combination that works with the existing bore measurements and their variances.

There is always out of round, its just a matter of how much. The out of round I'm dealing with is within factory spec, and works fine with the rings and pistons I had in there. Theres no reason I cant swap the piston to a forged and everything wont still work just like it was.

[acannell]

Would the pin being .011 bigger really make that big of a difference? That would mean you are also removing material from the small end of the rod potentially making it weaker, no?

No it would not.

Stiffness for all steel alloys is nearly identical.

Assuming the material remains the same and the wall thickness about the same, the increase in stiffness by going from .901 to .912 would be about 5%.

Tensile strength could increase drastically depending on the material but its pretty unlikely the stock pins are made of something substantially weaker than other pins, even upgraded ones.

[acannell]

That's a valid point! I haven't seen any issues with the .912 pins under 400hp, but I know several guys in the 400+ range that were trashing the pins and needed to move up on the pins to a thicker wall version. I know one guy was using the srt .866 piston pins and his were in a bad shape with only one season of racing.
We do sell the upgraded pins for people that plan to make mega hp, but they aren't cheap!

Third-hand anecdotals with no important details. This wouldnt even make it into an uncited wikipedia article about to be deleted. And simple calculations show it doesn't make any sense that a tiny increase in pin diameter wouldnt make any difference. In fact there it makes just as much sense that you may be weakening the stucture of the piston by doing so.

There are so many things wrong with that quote I literally cant believe anyone would use it to decide anything. How do we know that guys engine didnt have oil problems? What exactly were wrong with the pins? How well were his pins matched to his rods and pistons? What sort of racing was it and how much was there? What do 0.866 pins have to do with .901 pins?

Sheesh..is that seriously the standard of fact thats acceptable around here?

[Force Fed Mopar]

People make a lot more power on smaller pins. Nascar engines run a .787 pin now. Many SRT-4 engines make 400 on .866 pins. 03-04 Cobras are making stupid power on .866 pins also. Wall thickness and metallurgy is at least as important as diameter IMO, if not more.