Ring sizing and cylinder tapering - 1989 rering

[jbeary]

Okay 3.8 experts and others who may have run into the same issues, I need advice on a rering job.

First off, please don't berate me or think I want to do something half assed. In a perfect world, I'd already have my block down at the machine shop with no worries. I'm trying to rering a single cylinder rather than do a complete rebuild out of sheer necessity.

Heres my issue, the 89 FSM calls for compresion and second rings that are 2-5/64 and I think I could have a later model block or perhaps a correct block but a later rebuild since the Pistons I have require the 1.5 top and second rings. Anyone know when GM started using the 1.5 on the LN3's or perhaps just on the series 1 EV6 blocks? I know all of the SC series's 1 Pistons used the 1.5mm rings and some 95's used the newer series 2 1.2mm top rings. Block casting number lines up more with the 88 but isn't exact for either the 88 or the 89. (25532646).

Reason I need help is because the end gap on the 2-5/64 rings fall well into the tolerance range stated in the FSM. When I went to fit them onto my piston I realized they were too big and measured the existing rings and had to order a more expensive set of 1.5mm rings which fit the piston great but have a end gap that falls out of the 89 FSM tolerance range. This cylinder had no compression while the others were okay. Already ruled out valves. Have everything to reassemble the engine without going all the way and doing a full rebuild.

So today I went out to try and determine if the cylinder is out of round or is too tapered and needs a full bore rather than honing. I mean there's barely a perceptible lip at the top of this cylinder so I thought this would be a cake walk especially since there's no good reason I see I'd need a ridge reamed. The 1.5 rings measure a whopping .030 end gap at the top landing of the bore and then go down to .024 at the bottom of the bore. I don't see that the piston skirt is marred and this was a good running motor albeit one with a dead miss on #5. So now I'm worried about out of rounding and taper. Well some online experts say that's okay and the taper can be .005 while my taper seems to be around .0036 or .004 just to be safe. I'm taking the difference between the top ring land and the bottom of the bore and dividing that by 3 (not entirely pi but pretty close to get an idea).

That would mean I'm measuring the bore correctly since I come up with 3.8 exactly at the top. Actually a little under at the bore bottom. And that the taper is still in tolerance if it's at .004, correct?

Anyone ever rebuild an LN3 with the 1.5mm rings and if so were you also close to the same end gap measurement? The ASE chart I found online and other calculations say I can be at a .030 end gap and still be okay at the bore top. Sure smaller would be better, but without any real evidence of piston skirt wear, and because the other 5 cylinders showed good compression numbers, I think the rings just walked into alignment and there was some blow by going on too that supports that idea. Wasn't using or burning oil just no compression in that one cylinder. Unless someone yells at me, I think I'm just going to finish what I started with the single cylinder rering. Of course a light honing and Plastigauge are in order but I'm not planning to replace even the rod bearings at this point. Crank is not at all scored, neither are the bearings so I think those are okay to reinstall.

Would a full rebuild be better? Sure thing, just can't do that with this block if I even want to think of completing the SC rebuild I had planned with the 93 L67 I pulled from a bonneville ssei. Now I know I need new Pistons for that block and those are not at all inexpensive. Been considering a low milage L67 without the SC from eBay which would be $500 delivered but still no garauntee. Maybe a trip back to pull a part is in my future but future for sure unless I can find a sucker who'll trade a rusty Reatta roller for a good/rebuildable L67.

Thanks in advance.

[2seater]

I don't have a '89 FSM but I do know for certain that the original pistons in the '89 engine I have on my stand are 1.5mm/1.5mm? When I rebuilt it I used later model pistons ('95 I believe) that are 1.2mm/1.5mm. Frankly I wouldn't sweat the end gap too much. Much better than too little. Any idea what the other cylinder end gaps look like? The rest of the engine also has an undetermined amount of wear so my opinion is, do what you need to and run it until you replace the engine. I have had to replace a single piston on a couple of different engines, 283 Chebbie and 199 straight six AMC many years ago, and neither received any extensive machine work other than a bottle brush hone. Is it the best, of course not, but it will float the boat and you will likely never know the difference. I'm not quite sure why all the fuss about ring end gaps. The motion studies I have seen indicate the rings rotate in use although the original stagger may be maintained?

[jbeary]

Well that seals the deal for me. THANK YOU. I will start a new thread about piston alternatives for the SC rebuild when I can gather those funds to move forward.