Piston to Cylinder Clearance Issues

[jbuckster07]

OK before I go into details this is a little bit of the back story...

My cousin has had this blaster that has been in a box for like 6 months. He has been buying parts here and there and has finally got everything together, or so he says... So I drive 2 hours to Columbus, and tear this engine apart because the bottom end is toast, piston toast, head toast. This thing blew the &^%$ up lol. I get the motor apart, clean and replace everything. He bought new bearings for everything, new seals for everything so everything is going great! He got a weisco piston and we get this all done and get the top end on and it just looks like this piston is too small for this bore... I ask him if he had the cylinder bored to the piston, you can guess what the answer was.. NOPE! so this just ruined his whole day because I told him that there is no way that this piston is going to run in the jug without exploding in seconds. So I took measurements to see what you guys think about the size compared to bore:

Top of piston: 2.630" / 66.80mm
Botom of piston: 2.645" / 67.183mm

Top of cylinder 2.645" / 67.183mm
Bottom of Cylinder 2.648" / 67.259mm

Now I measured these over and over again with digital calipers. I know thats not the correct tool to use but it was all that was available to me. Now the clymer manual I have states that a NEW bore and piston should have a clearance of .0014"-.0016" with a service limit of .004". Now is that at the top or bottom. So with a piston clearance at the top of the piston and jug of .015" is WAY too big. and a clearance of .018" is even worse.

I told him over and over again before this build even started that he need to bore over the jug to the next size and bore it to a piston that he bought. I assumed he did this but i was highly mistaken. Now hes crazy mad that this isnt going to work and tried to blame me for it. I promptly told him where he could shove it if thats how hes going to act towards his builder lol.

Give me some thoughts and opinions regardless of what it is. thanks for reading!

 

[joeak47]

First things first...
you should not be concerned with the dimension (diameter) of the piston's crown, or top. The piston is machined a smaller diameter near the top (tapered) , because it will expand when it gets hot. It is closest to the combustion process. As a matter of fact... ALL pistons expand when they get hot. A forged piston just expands more than a cast piston.

Your also reading in that Clymer manual "Cast Piston" specifications and clearances, however your planning on fitting a "Forged Piston" (wiseco) to the bore of that cylinder. A forged piston requires more clearance than a cast piston, because it expands more than a cast piston when it gets hot and is at running temperature. Forged pistons also require a "warm up period", or they with give you that super groovy, ultra cool "four corner seizure" on the piston skirt. Helpful advice: Warm up your engine thoroughly before use.

That cylinder needs to be properly measured for diameter, taper and out of round. This is important!
A round, strait bore with no taper that if PROPERLY fitted with a piston is the heart and "soul" of the engine.
Digital calipers don't count. Get it properly measured with proper measuring devices. The calipers will get you in the ball park, but that's about it.
DON'T HALF ASS THE PROJECT!

You also want to measure the piston bout 10mm up from the bottom of the skirt.
Piston clearance for that wiseco piston should be "about .0025"