340 bore tolerance question

[pagilman]

:salute: Hi, I have end fitted my compression rings in a used 340 block .30 over. I have 3 cyclinders with a .003 bore taper from top to bottom. The ring end gaps were running from .017 to a max of .023 on a couple, but never more than .003 differance in any one cyclinder. My one book states .010 to .020 is acceptable on a small block, the same book states .013 to .023 is acceptable on the 340. My question is, do you guys think .003 bottom to top is excessive bottling? The pistons are flat tops that stuck out of the deck when I tore it down. thanks "your mopar brother Walt" :sad:

 

[demon seed]

I've known a few builders that want a little bit of taper so that as the piston travels up and down the rings essentially open and close, helps to clean them (at least that's the theory) but race engines want 0 taper. What that taper tolerance is for a 340 is not known to me but I wouldn't go more than .003". I'm sure one the other guys will chime in with the tolerance. We are talking taper ie. smaller at the bottom than at the top of the bore and not barreling right.

Terry

 

[C130 Chief]

.003 taper is quite a bit. Not ideal, and not what you could ever call a "new" engine. It should run fine for a while in a street driver however. As far as ring gaps go, just make sure they are no less than the minimum amount. You don't want the ring ends to touch.

 

[lead69]

Im not sure if this applies but if an engine was honed using torque plates it can skew the readings,Ive had a couple blocks that werent checking out right on taper/ring gap top to bottom.With the first one I asked my machinist and he showed how much distortion there was by bolting on a torque plate to my engine and then checking it,it came out right then and I will always bore/hone with torque plates..

 

[raymond]

If you are talking a total of .003" difference in ring gaps in a particular bore from top to bottom then this equates to around .001 of actual bore taper, which while not ideal is not a lot. I would put it together and forget about it. there were probably engines built at the factory in the sixties and seventies with those sort of tolerances.

 

[MtNemoMopar]

Your taper is fine.Straight bores will allow for tighter clearances though. Bore roundness is more critical. If it's apart, get a bore checker, and make sure those holes are round.If it needs a little boring, then you can take care of the taper as well,if it bothers you.

 

[moper]

As long as it's not tighter than .017 anywhere on the top ring, it will run fine. It will wear out faster, that's all.

 

[pagilman]

:salute: Thanks to all who have responded Im putting the crank in this weekend and the rods soon after as time allows. This engine will be used minimally for cruising to shows and sunday morning coffee, so i feel much better now. thanks Walt :thumbup: