Just rebuilt 787 top end now starter will bearly turn motor over even with plus out t

[realmechanic2013]

New help just rebuilt 787 and out 82.75 pistons in motor ran with 83mm before but when I rebuilt top end could see 83mm pistons seized so I honed block in put 82.75mm now starter will not turn I've even with plugs out

 

[mikidymac]

Wow that is entirely the wrong way to do it. The pistons are fitted to the cylinder by boring the cylinder then setting up the final clearance by honing.
So if it had 83mm pistons before and ran it probably seized from lack of fuel and/or oil not from piston size.
Now that you put undersized pistons in you have super sloppy pistons and it probably won't run the compression will be so low.
You could also have the pistons rocked side to side so much they are now wedged in the cylinder too.

 

[realmechanic2013]

No pistons fit same as other ones I measured cylinders and they were 82.92and pistons that were in it came out 82.50 even though they were 83mm

 

[realmechanic2013]

They fit good I think I needed to gap bottom ring

 

[mikidymac]

No. It doesn't work that way on these. You can't just file the rings to fit like on a car.
You need to have the cylinders professionally measured with a bore gauge then measure the pistons to the thousandth of the inch then subtract to find the clearance and see if it is within spec.
If the 83mm fit and ran like I said your 82.75 ones are wrong and it won't run.

 

[realmechanic2013]

The 83 mm seized all the way around the piston and broke the top off

 

[realmechanic2013]

I measured the cylinders they were coming out 82.92 and then I honed it

 

[mikidymac]

So did it have 83mm pistons before or 82.75?

 

[realmechanic2013]

83 but I know they were way to tight and seized

 

[mikidymac]

If it did how do you get 83mm pistons in a 82.92 hole?

 

[realmechanic2013]

That's what I'm trying to figure out ????

 

[realmechanic2013]

Maybe they were old ???? Out another motor

 

[realmechanic2013]

They only measured 82.55

 

[mikidymac]

Pistons will be stamped with their actual size on top. Are you 110% sure the arrow on top is facing the exhaust manifold?

 

[realmechanic2013]

1000% and I'll send you pics of old piston top. Stamp and my Mic meter on it

 

[realmechanic2013]

Pistons fit almost perfect .020 gap I think it's ring gape or lower ring ain't on spot it's suppose to be ...on lol ring pin rings are a bitch to get in hugs

 

[realmechanic2013]

Jugs****************

 

[racerxxx]

Piston wall clearance on a 787 should be .0045". That's the number you should worry about right now. Now, that's for a OEM piston. I think Weisco is .006". As far as other pistons like pro-x and others you should consult the mfg for the tolerance.

As far as the 83mm piston, I've seen aftermarket companies refer to them and round up. Most will just list the + oversize.

If your clearance is .020", you're almost 5 times over what you should be.


Mic the piston, use a bore gauge to check the cylinder dimension. Then start your math. Do not use a digital caliper or dial caliper to measure the bore, you'll never be close. The service manual covers where you should measure.

 

[realmechanic2013]

Found problem crank case was full of oil

 

[realmechanic2013]

Thanks everyone wish you all the best