marlinspike

I read conflicting advice. While my heads are off, should I bother cleaning off the carbon ridge at the top of the cylinder?

Fabrik8

Are you removing the pistons?

marlinspike

No, or at least I don't plan to yet. I understand that if I do, I will have to.

Right now I'm still trying to figure out the problem. Cyl 1 had low compression. I had a shop do a leakdown test and their opinion was leaking into the crankcase. Pulled the heads and the cylinder wall looks fine. I put a cup of mineral spirits Cyl 1 and 3 with each halfway up, and 40 hours later they had maybe gone down 30%. I tried this again in Cyl 1 with the piston near the top of the cylinder, still no significant seepage. I had the heads tested, and while the intake valves for Cyl 3, 4, 7, and 8 all showed leakage, the valves of Cyl 1 were fine.

My next step I think is to make a plate that will allow me to do a leakdown test on the piston with the head off. If it fails that, then I know the piston will have to come out.

Oh, and I'm using this opportunity for some porting and polishing of the heads and intake manifolds, boring of the throttle body, and porting out the exhaust where it meets the head (so it retains the step up as it goes from head to tri-y).

Fabrik8

Sealing with a liquid at ambient pressure won't tell you the whole story if you're having compression problems. You'd need a pretty bad sealing problem to show itself using that method. Doing a leakdown test is usually a better gauge because you have a slightly better approximation of dynamic ring sealing under pressure. You're trying to replicate a sealing problem that is occurring when the rings are moving and under pressure, which is a hard thing to do.

marlinspike

Yeah, I know, I just don't want to yank the piston until I'm sure, so I want to try a leakdown test using a plate instead of heads first. And I guess if that shows failure, I'll actually need to yank the whole engine and take it to a machine shop, since my understanding is that with an alusil block if I have to replace the rings I have to rehone the cylinder. I really hope you tell me I'm wrong about that lol.

Fabrik8

Yeah, you're going to at least need a hone with new rings, as long as there aren't any wear features that need to be bored out like a wear ridge. I don't know much about those cylinder sleeves though, so I don't really know what the rework process is.

Yep4doors

From my understanding wouldn't you want to remove all carbon while you're in there, or are we talking about something different than carbon build up?

It sounds like some bad rings though, or at least that's what the evidence had shown so far.

marlinspike

They're not sleeved - it's an Alusil block, so linerless aluminum that is then etched so the pistons ride on the silicon by itself.

The carbon is the ridge you get at the top, too high for the rings to touch. Some say sure clean them while you're in there, others say that it actually helps combustion to leave it.

Yep4doors

That's interesting, definitely something I didn't know.

marlinspike

Now I'm not so sure I need to rehone if I replace the rings. MB is pretty good about telling you everything you need to do usually, and all they say is if you replace the rings you have to measure the axial clearance and the cylinder wear with new rings. Hmm.....