Simple piston to cylinder wall clearance

[chilort]

So, I am building up a 2.5L CB for a daily driver. I'd really like to reuse the pistons if possible. Unfortunately, I don't have a set of '89+ FSMs, just my '86 set.

I'm also new to building the bottom end of an engine. So I have two questions.

1) What is the piston to cylinder wall clearance for a 2.5L CB supposed to be? (note: the '86 book says 0.0015-0.0025")

2) Is the spec for just one side or is that total?

I'm asking #2 because I took the engine to the machine shop today and the difference between the piston skirt diameter and the non-re-honed cylinder diameter was between 0.005 and 0.007.

[turbovanman²]

.005 is too loose for a cast piston, it will rattle and make alot of noise. If you really want to reuse your block, find a set of forged pistons, that clearance is perfect. I would set cast to .002-.003 max.

You measure from where the manufacterer tells you bust 99%, its the skirt to block.

[chilort]

Forged is waaaaay out of the budget for this rebuild.

[turbovanman²]

Forged is waaaaay out of the budget for this rebuild.

Well by the time you buy new cast pistons and bore out your block, your close to forged. If your bores are not tapered or worn, then you should be able to find some std forged and drop them in.

[chilort]

1) What is the piston to cylinder wall clearance for a 2.5L CB supposed to be? (note: the '86 book says 0.0015-0.0025")

2) Is the spec for just one side or is that total?

[turbovanman²]

1) What is the piston to cylinder wall clearance for a 2.5L CB supposed to be? (note: the '86 book says 0.0015-0.0025")

2) Is the spec for just one side or is that total?

I already said .003 MAX.

What do you mean one side? they measure the piston at the specified location, which is at the skirt for most pistons. They can measure the bore for taper or for being out of round.

[chilort]

Never mind. I talked to my machine shop and got it sorted out.

Edit: Turbovanman, sorry, I didn't see where you posted the acceptable spec.
And, what I was asking with #2 was, do you get 0.002 on each side of the piston or is that total for both "sides." As I now understand it after talking with my machine shop, that is total.

[4 l-bodies]

If you have .005-.007 piston to wall you most likely also have lots of taper in the cylinder walls, or someone previously had forged pistons in that block. IMO, do it right the first time or plan on doing it over again SOON...
If you do NOT have a decent budget for a rebuild, you'd have better luck looking in the salvage yard for something that was crashed and just transplant that motor into your vehicle. That would be WAY cheaper!
My advice should you decide to rebuild would be to get a very precise estimate at the machine shop for EVERYTHING. If you can't afford new cast pistons, what about all the other machine shop and parts charges? I spend more money on machine shop LABOR prepping the block alone than for the best forged pistons money can buy.
Sorry if this sounds negative, but you can easily spend a whole bunch of money quickly rebuilding a motor. You wouldn't be the first guy to not be able to pay his machine shop bill after everything is said and done. Then Murphy's law kicks in and you lose the tranny right after the motor install.
Good luck!
Todd

[turbovanman²]

^^^ Well said, it hurts but it really is the truth.

[chilort]

Is this SRT Forums? I must be confused or in the wrong place.

I asked two questions. Those two questions have answers that are facts. There is a specification in a book on piston clearance and also a correct way to calculate that clearance. Those two facts are all I needed to know.

If you wish to make guesses and assumptions about my knowledge or experience, well, you are free to do so, and being an internet forum, you don't have to keep it to yourself.

With all of this that was just shared with me through the power of the internet though I think I've decided to knurl the pistons, throw on some file fit rings, and toss some sand in the oil pan before I put it back together for good measure. I had no idea a machine shop actually charges for their services too. I thought they just did it all for free because they liked working on engines.

Seriously folks, give a person some credit.

[turbovanman²]

Is this SRT Forums? I must be confused or in the wrong place.

I asked two questions. Those two questions have answers that are facts. There is a specification in a book on piston clearance and also a correct way to calculate that clearance. Those two facts are all I needed to know.

If you wish to make guesses and assumptions about my knowledge or experience, well, you are free to do so, and being an internet forum, you don't have to keep it to yourself.

With all of this that was just shared with me through the power of the internet though I think I've decided to knurl the pistons, throw on some file fit rings, and toss some sand in the oil pan before I put it back together for good measure. I had no idea a machine shop actually charges for their services too. I thought they just did it all for free because they liked working on engines.

Seriously folks, give a person some credit.

Intesting response as 90% of the newbies do exactly what your being sarcastic about and then complain on my engine blew up or how much money they cost.

Your machine shop SHOULD have known how to measure pistons, if the're asking you, then they obviously don't know sh*t. Take a chill pill.

[chilort]

My engine shop did know. I stopped by there on a Sunday and asked because I happened to be in the neighborhood. I got a much clearer and quicker answer from him than I did here (possibly because I didn't ask the question well here to start with).

And how much does it cost where you are to have a block bored? Cast pistons with rings from Cindy are $160 and forged are $450 w/o rings. I don't mind going with over sized cast pistons, that actually is in the budget, though it means I cut down on the "other" parts I upgrade (i.e. ported exhaust manifold).

Not everything running has to have forged pistons. The donor for this motor was an '89 mini with 150k miles on it (though I was surprised to see the evidence of detonation on the edge of the pistons). I pulled the engine myself and it never had forged pistons in it. The current engine in the C/S has over 140k miles on stock pistons. There are plenty of turbo Mopars (more so than not) running around just fine on cast pistons for tens of thousands of miles. If someone is going all out, then yeah, go ahead and put in forged. But I made it clear with the first post that this is for a daily driver, not a race car. If I want to go fast I'll get in my truck or work on the Superbee.

Like I said, I asked two questions that have factual answers. If I were to have said, "Hey, I'm building up a race motor that I plan on running 25lbs of boost and running up to 7000rpm," then go ahead and question the viability of running cast pistons. The automatic trouncing of someone's engine plan without actually knowing the plan is really annoying. Maybe ask the plan first and don't assume.

[turbovanman²]

I threw out forged pistons as you had excessive bore clearance, my bad for trying to save you some money.

I am done with this thread.

[4 l-bodies]

Even a daily driver needs to be mechanically sound. You tell us your new to rebuilding the bottom end of motors. You tell us forged pistons are WAY out of your budget, you tell us you would like to re-use 150k mile 2.5 pistons (even though they show signs of detonation). These are not assumptions, this is info you gave us. It tells us you are trying to build a motor with little knowledge and on a shoestring budget. Then you get perturbed when two knowledgeable Mopar engine builders try to give you some good free advice? Please pardon us for annoying you.
Do it the old fashioned TD way, splurge and throw in some NEW hypereutectic pistons and file fit rings in that standard bore along with those gently tapered and out of round cylinder walls and call her good. With the cost of new main, rod, and aux shaft bearings, oil pump, pickup, one might consider re-using those too, after all, they only have 150k on them. Pop for some new gaskets and paint, in other words, slap some lipstick on that pig, then call it rebuilt.
With all that money you saved, splurge for that ported exhaust manifold, that ought to make it run super.
Good luck, your gonna need it.
Todd

Is this SRT Forums? I must be confused or in the wrong place.

I asked two questions. Those two questions have answers that are facts. There is a specification in a book on piston clearance and also a correct way to calculate that clearance. Those two facts are all I needed to know.

If you wish to make guesses and assumptions about my knowledge or experience, well, you are free to do so, and being an internet forum, you don't have to keep it to yourself.

With all of this that was just shared with me through the power of the internet though I think I've decided to knurl the pistons, throw on some file fit rings, and toss some sand in the oil pan before I put it back together for good measure. I had no idea a machine shop actually charges for their services too. I thought they just did it all for free because they liked working on engines.

Seriously folks, give a person some credit.

[overlordsshadow]

Rank, but funny thread. I think signals got bounced and wires crossed as everyone is taking offence from what others have said.

Good luck Chilort, thanks Simon and 4 lbodies.

[chilort]

I was actually trying to avoid all of the "advice" on how to build my engine from all of the experts here by saying that it was a simple question and that it was for a daily driver. I don't know why people can't just answer the question without prattling on about what they think they know about an engine they haven't even seen.

Just because pistons are old doesn't mean they are garbage. I was asking about the specs on a CB because I didn't have the specs for a CB. I've spent most of my time working on big block Mopar race motors that are built a little looser and I thought maybe the turbo cars were built a little looser too. While this spec is looser than the non-turbo cars it wasn't as loose as what I was expecting.

If old pistons are garbage then I'm in real trouble because the 40 year old stock flat-top slugs are still what's in my truck. Of course, the ole truck just runs and runs


I almost hate to get mean like this, but I have a couple of points
1) 4 l-bodies -- do you have a reading comprehension problem? If so, let me help you. I actually AGREE with you that a set of over sized cast pistons may be what I need. I didn't know the spec, that's why I was checking here. I have a fixed budget and have planned for several contingencies. If something costs more then planned, then the least necessary parts are the first things that get hacked out of the budget. Now, if I were to have said that I was planning on throwing the old bearings back in, well... then you'd have a point.

2) While I might be new to turbo engines, this is not the first engine I've had my hands in. And unlike some of the experts here, I've never windowed a block.

Here:
http://www.turbo-mopar.com/forums/sh...223#post350223
I've started a new thread so people can make all the assumptions they want and talk about how you can't build a proper engine unless it has 4 bolt mains, forged pistons, ARP head studs, a knife edged crank, and what ever else makes ya happy.

[Speedeuphoria]

If old pistons are garbage then I'm in real trouble because the 40 year old stock flat-top slugs are still what's in my truck. Of course, the ole truck just runs and runs

So whats the dynamic compression ratio on that truck? How much boost is it running?

In all honesty I this statement made by you in the original post is why people were giving you advise:

I'm also new to building the bottom end of an engine.

You set yourself up, its in people's nature to try to help others by giving advise of there own previous mistakes. I'm just sorry that you dont open your ears a little more, no one is telling you what to do just giving advise. I hope you dont come crawling back with your tail between your legs in the future