A Question For The Engine Gurus

[rv6aflyer]

So I'm working on an N42/N42 L28 rebuild.  I've read several places you should always put pistons back in the same cylinder they came from.  I can understand that you want to keep the complete rod together, but I can't figure out why you need to put the piston back in the same cylinder if you hone the cylinder and replace the rings.

 

Any engine gurus out there who can explain if and why it is necessary to put the piston back in the same cylinder even when you have the cylinder honed and use new rings?  Thanks.

 

[Diseazd]

The rods are numbered 1 through 6 on the big end.......when the caps are installed, the numbers will be on the same side. Why wouldn't you put them back in order? My guess is it would work out of order, but why?

[240260280z]

Bores are matched to pistons for correct clearance when initially bored and honed.

If you mix the pistons up and can keep these optimal piston to bore clearances for all 6 then you should be good to go.

You can knurl smaller pistons to sort of meet the clearance requirement but it is not recommended.

Edited March 17, 2015 by Blue

[siteunseen]

ZX pistons have a weight code stamped into them, keeping them in order helps internal balance.  In my opinion but I'm no "guru". 

[CEgg11]

I agree with siteunseen. This is a factory-balanced engine, which is the biggest reason you'll want to keep them all in the same order. I don't think bore has as much to do with it as weight balance.

[Diseazd]

 

I agree with siteunseen. This is a factory-balanced engine, which is the biggest reason you'll want to keep them all in the same order. I don't think bore has as much to do with it as weight balance.

 

Now that would make sense......However did the factory really balance to each engine or try to equalize the weight on all piston / rod sets?

Edited March 17, 2015 by Diseazd

[CEgg11]

The engines were individually balanced, cranks drilled out and rod/piston sets matched. The flywheel and front crank pulley are individually balanced as well.

[Diseazd]

The engines were individually balanced, cranks drilled out and rod/piston sets matched. The flywheel and front crank pulley are individually balanced as well.

Then you've answered his question.......keep 'em in the holes they came out of. That would explain why the rod big ends are numbered.

[rv6aflyer]

So this really raises some questions about using replacement pistons, say for any engine that gets bored out. I see posts about using different rods with different pistons and nobody seems to think much about it. If it is a balance issue, then the new piston/rod combinations could be balanced and it seems like you would be good to go.  If the combination of crank and pistons/rods needs to get balanced as an assembly, that would be a much bigger deal and I don't see any references to people having this done.

 

So if someone replaces the stock pistons with forged pistons (which I assume are lighter), does that screw up the crank counterbalances, or is the crank balanced by itself and as long as the piston/rod assemblies are balanced, everything is ok?  I hope I'm not sounding ornery , I'm just trying to figure out what can be safely done and what is a problem.

 

The reason for all these questions is that one of the pistons I was planning to use (flat top) has a cracked skirt.  I was going to try to get just one piston, and then balance that against the other piston/rod combinations.  I'm finding this an interesting discussion.

[CEgg11]

I've done engines with replaced pistons. Just have a machine shop balance them as a set. Shouldn't cost more than 100.00, and all will be well. Please keep in mind, since this is an L motor, the concerns and worries of balancing just aren't as worrisome as a V motor, where you have mass working on opposing planes. Everything is, well, in the same line.

[Zed Head]

 I've read several places you should always put pistons back in the same cylinder they came from.

What are these places? 

 

It seems sensible, since putting things back the way they came apart is generally a good idea.  But you could also rationalize that moving them to other bores, as long as the piston specs. match the bore specs., is a good idea.  The wear spots would probably be slightly different, extending the life of the engine.

 

Just saying, if you were starting from scratch with used pistons, you'd take the measurements, including weight if that's spec.'ed, and fit the pistons to the bore they match best.  If you're rebuilding, then all of the pistons are probably out of spec. after honing anyway.

 

Whatever you do, you still have to take the measurements and hit the specs.

[Patcon]

Since the pistons are numbered I would assume the rotating mass was balanced so keeping them in the same hole maintains that balance. If you move some or replace some I would want to have the rotating assembly rebalanced. then there is nothing to worry about...

C