Piston the cylinder clearence on a 97mm 426 question.

[SWIGIN]

I have heard guys on here say that they like to run there piston to cylinder clearance a little tighter then what is recommended. I have a 2000 YZ426 that I am boring out to 97mm and I have a JE high comp piston ready to go in. Since I can't find an answer in the search is, What clearance should I ask for with my new set up?

Thanks

Edited March 28, 2012 by SWIGIN

[grayracer513]

In 9 years on this forum, I don't ever recall hearing anyone say they ran tighter than recommended piston clearances. You should run whatever the piston manufacturer calls for.

[SWIGIN]

The site search did not bring up anything but good old Google did.

https://www.thumpertalk.com/topic/256255-yamaha-yz426-performance-mods/

I knew I seen it, and normally I would never go against what the piston manufacturer calls for but I figured I'd ask.

[grayracer513]

I see that post as a recommendation against a other than recommended piston clearance, not the other way around. Of course, he didn't specify what the piston manufacturer's original recommendations were, either. The clearance he ended up using was pretty typical sounding to me. Did I miss something?

Increasing piston clearance reduces the size of the contact area of the skirt, and thus increases the specific load on that surface, and therefore also increases the likelihood of a failure of hydrodynamic lubrication. It also reduces the heat transfer from the skirt to the cylinder wall for the same reason. Overall, tighter is better, and a running clearance of between .0008 - .002" at operating temperature is generally considered ideal. The cold clearance given by the manufacturer for a given engine is based on this, and intended to produce a clearance in that range when warmed up. Cylinder construction and piston materials will have an affect on cold clearance.

[SWIGIN]

The way I read it, he was saying that when he first rebuilt his engine he went with .003. Then after talking to some builders decided to go with half that (.0015).

.003 is what I have seen manufactures give for forged piston specs.

I'm new to these engines so i was just wondering if there was something to this.

[grayracer513]

I've seen some piston manufacturers recommend that, too, but not nearly all do, and I don't especially approve of running that much bore clearance. Consider the fact that the OEM piston is also forged, and doesn't fit at all that loosely. At .0008 - .0018", it's less than half that. The service limit is .004", so you could run one that loose, but why start at .003"?

.003 is looser than the last high performance 383 Chevy I built with 4" forged TRW's.

[SWIGIN]

This is why I asked, .003 sounds loose to me. I asked for .0017 so we will see how that works out.