Piston to cylinder wall clearance

[NONHOG]

So I have a Oldsmobile 455 @ a machine shop now, and he calls to tell me the block is bored/honed 4 thousands instead of 2. They are forged and all the responces on ClassicOlds site so far has been "4 at least, no problem"
The machine shop owner says it may have piston slap.
The manufacturer calls for 2 but the Olds guys say go 4.
What would you experianced engine builders do?

Anyone here own a machine shop?

[Ticker]

Specifics would help, but if the piston manufacturer wants .002, I'd sure use .002. It's fair to say that it's not unusual for a forged (not cast or hypereutectic) piston to be run at .004 or even a bit more...but IMHO those are racing clearances and I would expect some slap at startup.

I would say that it's a pretty big whoopsie to miss a bore size by that much.

My suggestion is that the shop pay for a set of custom pistons that are exactly the same as what you've got...but .002 bigger. Depending on the brand it's not that much money - and it's their mistake.

I guess in the end I'd say it's not the end of the world, but if it's a spendy build (and I have to think with forged it is) I would take an easy route to make it right with some custom pistons.

Who's the shop?

[NONHOG]

I should clarify it was bored/honed before I got it. Came with pistons also.
I took it to a shop to check it over and thats the report.
So the only thing I can do is live with it or bring in another block.

[Ticker]

Ah, understood. Not sure if you got it from an individual or someplace retail, but those pre-bored/honed blocks aren't a great deal for exactly this reason.

You also need to get the details on the surface roughness average (should be on paperwork somewhere as Ra) as that will determine the type of ring the block was honed for. Same for the deck surface - a thin gasket requires a very smooth block deck. Then there's deck height...

It's not a black-and-white call. If it's a daily driver, then I'd say pass and get a new block. If it's a mid-level performance build on occasional driver, keep the block and don't sweat it. If it's a spendy build or you're planning on any power-adders, then get another block.

Or, again - custom pistons. These are surpisingly cheap and can be had in a week or two. Someone will want the forged ones you've got. That's probably overall the cheapest way forward.

I take it everything else checks out on the block (mag, pressure test, etc.) and the other block dimensions (CR, crank, etc.) all work for your design?

[68gmsee]

I don't know much about that engine but when I looked at the specs for a big block chevy, the .004 is at the max end but still within allowable limits.

Also, as Ticker mentioned, it depends on what you're going to use the engine for. If you're going to run it, the info I find is that forged pistons require a greater clearance. It may not be a game stopper yet. I'd be tempted to run it and see what it sounds like.

[Super73]

I recently had an out of town shop do the tolerances on my new 418. I asked specifically for the motor to come to me disassembled so I could double check their work since I'd be spraying a big shot of N20 through it. Pistons came in at .0035 clearence and rings were gapped .018 and .022 (I asked the rings not be gapped). Called the piston mfg and they told me the pistons should have min .0045 clearence and rings should be .027-.028 top and .029-.030 on the second. The company that did the motor does LS motors everyday, but either failed to realise their mistake or had a different idea than the piston mfg. The bores got straightened out locally and I regapped the rings.

In the end, I think you should contact the piston MFG and see what they recomend for your intended useage. Now, I'm not saying the people at the machine shop are idiots, but the MFG of the parts you are using should have a better handle on it than anyone else out there. Or at least I feel they should.

[NONHOG]

Couple of concerns,

One is the manufacturer suggests .002 clearance.
Which all the Olds guys, who build these engines say .002 is too tight and it will cause scuffing. Many of those on the Olds site have many builds under there belt. They have done it both ways.

Second is piston slap, if its just cold I think I could live with that.

[Ticker]

Maybe to net it out - it ain't gonna explode with .004 clearance You may have more blowby, and you may have a bit less power, and you probably will have some cold piston slap...but in the end maybe we're splitting hairs.

[truckdude239]

worst case is have the cylinders sleeved and rebored

[Ticker]

Sleeving a cylinder is about $150-200 each. $1,600 would be a lot of jing to fix a problem that has many cheaper solutions

[Super73]

Just spray a bunch of N20 on it.. Then the cleaernce will be acceptable..

Really though, I don't think .004 is going to cause a major issue if you let it warm up before putting a load on it.

[truckdude239]

i'd look long term what is the maximum clearance allowed? if you already got .004 and the max wear is .005 that motor wont last long

[chevybuldr]

Is this a street motor? Also was the person you bought it from using it for racing or was it just to be sold? Rings you will have to get file fit for sure there. Whose piston is it?

[NONHOG]

From what I've been reading on the Olds sites more people have had problems with scoring and overheating running .002
Though Speed Pro rec. .002 most have run .004 or more w/o durability issues.
I do plan to race this engine and may see a small shot of N2O.
It will mostly be a street engine.
My machinist mentioned how much these pistons move (grow) so would that not be counter to a small clearance?
I wont mind warming it up, probably wont hear any piston slap with my Pypes
RacePro mufflers anyway.
thanks all for the reply's !

[Ticker]

Yep, forged grow significantly which is why they typically run wider clearances; cast are typically fit to .001 -.00025, hypereutectic typically .001 + .00025.

Time to stop worrying and just go for it

[chevybuldr]

You are good then. A racing application and NOS see more heat and abuse so they need more clearance. Dont worry run it. Heck some of the high end JE and Ross stuff want has much as .007. Everything is different. Dont worry run it. Just remeber you have a ring gap to set. No out of the box stuff.

[RaysK20]

Are they the speed pro power forged pistons with the black moly coating on the side? If so .002 is the correct clearence. I had a similar ordeal with a mopar 440. If they were the old school type with no coatings on the side I would say .004 is in the ball park. The make a bit of noise at cold start but should go away pretty quick.

[NONHOG]

Originally Posted by RaysK20

Are they the speed pro power forged pistons with the black moly coating on the side?

Nope, no coating ...."old school type"