Piston To Wall Clearance

[Dodge72]

So I just got my 360 block back from the machine shop from getting cleaned, shot blasted, hone, bore, and new plugs and cam bearings installed. Beautiful work! Anyways, I didn't know how much they would need to bore the block (standard bore originally but had rust in the cylinders). So I held off on ordering pistons. They get it done and tell me they had to bore it .030 over. Okay great, ordered Sealed Power H116CP30s.

I had somebody on FB ask me if I brought the pistons with the block so they could bore to proper block to cylinder clearance. I did not because I wasn't sure how much they needed to bore. The machine shop didn't mention anything either (and I trust them that they would've mentioned something). Now I'm concerned that I missed a vital point (first time tearing down an engine and rebuilding it completely with new things). Looking up specs for my pistons I can't find the clearance specs they need either. Where do I find this info? Does this vary between piston brand to piston brand? I sure hope I don't need to bring the block back to them and spend more $$$ on this to get redone. Any help is much appreciated!

 

[RustyRatRod]

You didn't miss anything. Modern automotive pistons are made to more exacting tolerances than they used to be. Years ago, they'd mic a piston, bore a hole rinse and repeat. That way, they could set the clearance up in each individual cylinder. This is still done today in pretty hot street and race engines where every bit counts. When you get the pistons, get a nice set of feeler gauges....long ones and see what clearance you have. Most competent modern machine shops don't need the piston to bore a block. Some people still insist on it and that's a personal choice. I sent the pistons to the shop with my slant block recently, but that was because I am using another type engine's piston in the block. I am using the Chrysler 2.2 piston and its standard bore is "about" .045 over from a standard slant six bore, so the pistons were necessary. Just measure and see what you have. Hold the piston by the pin, stick it in the bore with a .0015" feeler gauge and see if it will go. If not, try a .002". IMO anything past .003" will require a call to the machine shop and get them to bore it to .040 after you swap the pistons out. But this will be rare. These figures also are for cast pistons. If you are using a forged piston, I'd say no more than .005", because forged pistons expand more so they need more clearance. I am confident you will be fine.

Oh and when you measure using a feeler gauge, the feeler gauge should be able to be pulled out using what I would call a slight to slightly moderate drag when you find the right one.

 

[Dodge72]

Thank you Rob! I will definitely get clearances checked out when those pistons get here. They are hypereutectic pistons so I'm not sure if that qualifies as cast (I know they aren't forged).

 

[RustyRatRod]

Thank you Rob! I will definitely get clearances checked out when those pistons get here. They are hypereutectic pistons so I'm not sure if that qualifies as cast (I know they aren't forged).

That is a high silicon cast piston. Speed Pro "says" they do not require special ring gaps. I disagree with that. I recommend you follow the Keith Black recommendation for ring gaps, which will require you to file fit the rings to each cylinder. A little extra trouble but, IMO better safe than sorry. I believe if I remember correctly, the formula for a naturally aspirated street engine is .006" of gap for every inch of bore, so that would be "about" .024" top ring gap. The reason Speed Pro does not recommend special ring gaps is because they do not use as high a content of silicon in their pistons as KB does, but why take chances? This is of course just one peon's opinion.

 

[Hellrats]

I have used those pistons many times. The spec sheet will be in the box with the pistons. They are a factory replacement piston. The rings will be with them as well. I have done three 360 engine in the last year or so using them and all of the ring end gaps were correct right out of the box. But like Rusty says, check and make sure. I think you'll fine them correct. They are a good choice, nice pistons for the money.

 

[Dodge72]

Awesome awesome. If (or when ) I build another engine I'll keep clearances in mind if it get crazier than this one. And yes I'll still be checking all ring gaps for sure, that one I knew well. Thank you both!

 

[brian6pac]

That is a high silicon cast piston. Speed Pro "says" they do not require special ring gaps. I disagree with that. I recommend you follow the Keith Black recommendation for ring gaps, which will require you to file fit the rings to each cylinder. A little extra trouble but, IMO better safe than sorry. I believe if I remember correctly, the formula for a naturally aspirated street engine is .006" of gap for every inch of bore, so that would be "about" .024" top ring gap. The reason Speed Pro does not recommend special ring gaps is because they do not use as high a content of silicon in their pistons as KB does, but why take chances? This is of course just one peon's opinion.

I talked to a guy at KB and he told me that the top of the piston on a hyper is smaller than the skirt by more than .020 exposing more of the top ring, so the ring gets more heat that's why the big end gap on the top ring only the second ring is normal gap.

 

[Garrett Ellison]

All good points and valid advice. Being that a good hone job will use torque plates, the cylinders won't be true and round unless the heads are on. Some builders advise using a tapered type fixed bore ring compressor or a precision fixture made from a piece of stock that is the same as the bore size to size the ring gaps. In my opinion, FWIW, the direct measurement technique with the feeler gauges and light oil to check the piston to wall clearance is as good as it gets.

 

[RustyRatRod]

I have used those pistons many times. The spec sheet will be in the box with the pistons. They are a factory replacement piston. The rings will be with them as well. I have done three 360 engine in the last year or so using them and all of the ring end gaps were correct right out of the box. But like Rusty says, check and make sure. I think you'll fine them correct. They are a good choice, nice pistons for the money.

I agree, but as they say......ESPECIALLY with something like this, check check and tripe check. Who wants to pull "all that" back apart again? LOL