Std. piston should fit std. bore? You can buy big bore jugs over the counter without pistons and vice versa.

Jim

 

If you are talking MoCo pistons/cylinders, the clearances can vay greatly. I would never install a big bore kit without having the piston to bore fitment confrimed by a qualified machinist with torque plates.

A piston, forged or cast, should have the "fitment" built in by the manufacture. In other words, cylinders bored to 4.125" should accomodate a piston manufactured to fit a 4.125" bore. However, IMHO, it is always a good idea to have the machinst check the piston dimensions before boring cylinders. Buy, you do what you want.

Just because you can buy pistons separate from cylinders doesn't mean the fitment will be what it should be. Like Reagan used to say, "trust but verify".

 

Thanks, I think I will reconsider buying prebored cyls

Jim

 

if your buying Harley pre bored cylinders any piston you select from Harly will work its only the aftermarket pistons that need to be verified for size.

 

It is always a good idea to check tolerances of the rings and test fit each piston to the cylinder prior to install to find which fits best where.

Drew

 

cyls. are bored to a +/- tolerance...

pistons are made to a +/- tolerance

you dont want to match up minimum tolerance pistons to maximum tolerance cylinders, and viceversa...

you gotta get them anyway, so do it right, and match cyl. to piston...

 

Have to take exception to this statement. It's like xcvator says; the MoCo manufactures, or has manufactured, pistons and cylinders to a +/- spec. You buy a big bore kit which includes pistons and cylinders that are randomly matched up as a "kit" without benefit of checking piston to cylinder fitment. So, you get pistons on the - side of the spec and cylinders on the + side of the spec and that combination is likely to either be noisy and/or consume oil. I won't even get into the issue the "roundness" of the MoCo cylinders.

I don't mean to just lay this a MoCo problem, same applies to any piston/cylinder combination. Will you get lucky and get a good tight fit; maybe, maybe not but why take a chance?

 

We measure the skirt diameters, with either Starrett, or Mititoyo micrometers(we have them from .500" models, up to 12" units).
At that point, we dial the specified piston/cylinder wall clearence into the thimble, and lock it.
The micrometer is then held in a machinist vise, and a Mititoyo Dial Bore guage is "0"ed, EXACTLY, to the micrometer. No guess work, no faulity readings.
Once that guage is set, then we then bore the cylinder to within the measuring range of the bore guage, and we leave .003"-.004" for torque hone,( 1.500" thick, 6061-T6 aluminum custom torque plates) and the same guage follows that set of cylinders to the hone.
That is how we perform that work.
Scott

 

Y'all some smart ****'s....All this little bits of information will eventually fill my brain and mybe i can put it to use....LOL....Ain't this a great place....Smart ****'s.

 

I agree. Each piston should be fit to it's bore.....same for rings....gap and file fit rings to their "assigned" cylinder.

If you're going to take an engine apart...best to do it right, IMHO.

~Joe