Piston-Bore clearance

[BigBlockBabe]

Just picked up my 383 block from the shop, where I had it tanked and honed for a re-ring.

But it seems they went a little overboard with the power hone; now the piston to bore clearance is .010, which is over the .008 that the book calls as a max.

The ring end-gap had been .017 before, and now it's .023, which I figure means they overbored it about .002.

Anyone have any idea of how bad it would be to run it like this? I had hoped to save the old pistons, as the taper had been minimal, but now I am concerned about the loose pistons. Besides the noise when cold, what are some problems I might experience?

[djswwg]

You're guaranteed to have excessive blow-by, burn oil, and likely suffer a slight piston slap noise. Also it'll cost you horsepower and wear out the rings, pistons and cylinder walls too fast. I'd buy new 30 over pistons, then take them to a different machine shop that knows how to measure the pistons and bore the block accordingly. Remember, forged pistons need a couple thou. more clearance than cast. Good luck...........djs

[BigBlockBabe]

I'm hoping I got the measurments wrong! This is the method used:

--installed piston
--measured with feeler gauge perpendicular to wrist pin
--was able to wedge a .012 in between

Is this the right way to do this? The skirts feel snug going in, which I'm not sure means anything.

To add to my confusion, I have another 383 that has not been machined and the clearance above the wear ridge is .015. What's going on?

Thanks.

[DartGT66]

The points where the clearance should be measured may vary from one piston style to another. With too big a clearnce, the ring lands will get a lot of hits and will eventually fail. I would take the block & pistons to a machine shop to be measured.

[BigBlockBabe]

I'm using the original (stock) pistons, if that makes any difference? They have the little dish and come up to the top of the deck.

[BigBlockBabe]

DartGT66, what you said about ring lands hitting helped me figure out what I was doing wrong.

I was measuring at the ring lands instead of the thrust face. So, the clearance is .003 on a really cold block. The Taylor book calls for .008. I'll double check tomorrow but it's looking good.

Thanks for the help.

[djswwg]

BigBlockBabe; measuring piston to bore clearance with a feeler gauge is about as accurate as measuring your 1/4 mile time with a calender-not very! You need a bore micrometer and a proper vernier caliper to first check bore size, then piston o.d. The difference between the two is the clearance. Now, as these tools are quite expensive, only automotive machine shops have them as a rule. Now you know what you have to do.................djs

[Billydelrio]

I always use my long feeler gauges to measure piston to bore clearances. I have good dial bore gauges, inside, outside, and some other special micrometers, but the long feeler gauge is the way to go on piston to bore clearances in my opinion.
BigBlockBabe - The long feeler gauges are not expensive and are easy to find in .0005, .001, .0015, .002, .0025, .003, .004, .005, .006, etc. These tools will get you a good measurement as far as automotive piston clearances are concerned. The dials and micrometers will be more exact, but unless you need more exact measurements, why spend thousnads of $ on them.

To use the feeler gauge: I place the feeler gauge into the clean cylinder and then slide the piston in. The long feeler gauges are long enough to hold on to. If a .0015 is free, but a .002 is a little too snug, then you know the clearance is between those two measurements.

The desired clearance depends on the piston's design and the intended use of the engine. Boat engines seem to like more clearance than car engines.

Billy