Inliners International Forums General Discussion Engines 270 Piston to Cyl Wall Clearance Question

Hello All,

Need some advice from the engine gurus.

Am moving along on my 270 rebuild project. This is for my GMC CCKW 2 1/2 ton truck. Stock engine configuration, no hop-ups. Engine was just bored 0.030" over (up from an arsenal rebuild of 0.020") to clean up the cyl. walls.

Provided my machinist with the pistons that I plan I using. They are NOS, but I am unsure of the brand as I purchased them second hand. They are definitely cast, not the heavy Zollner forged like ones that the military uses. Full skirt, three 1/8" rings, and one 3/16" oil control ring.

Machinist said that pistons were uniform in diameter, so he sized all bores with 0.002" clearance.

I double checked all measurements and found that there is almost 0.001" variation among the pistons (as measured at the bottom of the skirt - perpendicular to the wrist pin axis). There is also about 0.0005" variation in cylinder diameters.

So theoretically, I could end up with a cylinder with just under 0.001" piston to wall clearance.

Army TM specifies min. 0.0035", and even provides a separate warning about never going below this value.

I realize I am not running the OEM forged pistons, but 0.001" - 0.002" seems a little tight to me. Am thinking about taking the block back, and having him open up the bores another 0.0015 to 0.0020"

What do you guys think?

What piston to wall clearance do you typically run on these engines?

Thx in advance for any help that you can provide!

Cast pistons do not have the same expansion as forged units do. The clearances can be kept tighter.
With that said. newer design cast pistons can be run tighter than old design cast pistons. His .002 sounds reasonable.
I run .004 on my Ross forged pistons.

I agree with tlowe. The pistons you’ve described sound like those available off the shelf – for example: Egge 270 GMC which call for .002 clearance. As for the variations you’ve mic’d don’t forget to divide those measurements by two. If the largest piston is .001 larger than the smallest one – it will tighten the clearance by .0005 on opposite sides of the bore. Same with the bore variation.

Let’s review the arithmetic just to make sure we’re on the same page. Stock bore is 3-25/32 inches or “3.78125. 30 over would be “3.81125. An off the shelf 30 over piston designed for .002 clearance should therefore mic at 3.81125-.004 = 3.80725 somewhere low on the skirt (just as you described – perpendicular to the wrist pin) typically some set distance from the oil ring groove.

What are the measurements of the largest and the smallest pistons?

What are the measurements of the largest and smallest bores?

The solution during the build may be as simple as rank ordering the pistons from largest to smallest – and similarly rank ordering the bores from largest to smallest – and then putting the corresponding piston into the corresponding hole . . .

Thanks to all for your input.

Yes. I did try moving them around. Basically I end up with 2 good (assuming 0.0020" is acceptable), 2 not so good, and 2 pretty tight:

Min Cyl Dia. 3.8112 3.8113 3.8113 3.8114 3.8115 3.8115

Piston Dia. 3.8090 3.8095 3.8100 3.8100 3.8105 3.8105

Clearance 0.0022 0.0018 0.0013 0.0014 0.0010 0.0010

I have pretty much reconciled myself to the fact that I will be going back to the machine shop. The question is just how much clearance to shoot for.

While this is not a performance motor, it will be pulling a relatively heavy load around (truck weighs 10K lbs.) So, it seems more likely that it will incur higher piston temperatures than, perhaps, a normal PU truck.

I know the cast pistons, especially the hypereutectic alloys, expand less than their forged or less-alloyed counterparts, but 0.001 just makes me feel uneasy.

Thx again....

Based on these measurements I agree the bores are too tight . . . And the machinist had your pistons in hand when they machined the block for .002 clearance?

That is correct. Thinking they maybe spot checked the smaller ones and assumed that all were the same.

Will be talking to them again soon.

Thx again for the input.

Just checked some new older design cast pistons for a 218 Plymouth 6 and they call for .0015 clearance as measured .060 up from bottom of skirt. It has a smaller bore of 3.250 and taking it out to .030