Ok... I've never done a full engine rebuild... replaced heads, etc.
When does a cylinder need to be rehoned or bored?
Should the walls be glassy smooth or somewhat rough? (only asking, another thread mentions sandpaper?)
does it differ between 2-cycle and 4-cycle? I have a snowmobile that's reading 50psi on one cylinder while the other is 155lbs
Definitely not glassy-smooth. Use a dingle-ball hone, lubed with WD40.
Measure the cylinder for wear, or look for scratches. You may need the skill and opinion of a professional here. Also, the ridge may be an indicator of wear.
Cylinders normally have a taper to them after a ton of miles. It's a good idea to run a bore guage down them and see how straight or out of round they have gotten. 50 lbs of psi in a cylinder is not good. There's a problem there somewhere. If you know what engine it is, you can probably find specifications for it online.
FWIW, it's not that big of a deal to get an engine bored/honed correctly at a machine shop. My guy here in FL does it for $25 per hole. Any hole, any size. is the joke.
Interestingly enough, it was more cost effective to bore out a spare block than deal with a seized water pump on my Saab engine...that's how cheap.
Per
I've got 2 engines.
the Rotax 583cc is the one I'm most concerned about right now.
the other is a Suzuki GS750.
Are the dingle-ball hone's fairly easy to work with?
Boring of the cylinders may be cheap. It is the expense of new pistons and rings that add up.
The dingle ball is really easy to use. All you need is an electric drill. And some oil.
With the dingle ball hone, it's all in how you stroke it.
A bore gauge and a shop manual should tell you whether you need to bore out and do rings/pistons. There's will also be a ridge at the shop of the cylinder, since the rings don't travel to the very top of the hole.
Here's food for thought: I don't like to replace a head gasket on a motorcycle engine without also doing the base gasket, since the un-torquing and re-torquing of the head also means doing the same to the base gasket. So, since I'd plan on replacing the base at the same time, I'd further suggest that you'd do well to remove the bores and take them to a competent machine shop for their diagnosis. If they're straight shooters, they'll tell you if you're okay without a re-bore - since you probably are. At that point, hit the cylinders with a ball hone, wipe 'em clean, and re-install.
(For the record, motorcycle rings can usually be compressed with your fingers, no need for a special tool. If they're stubborn about fitting into the cylinder at that last little bit, the cap of a Bic style ball point pen (the pointy-ish clip part) does a great job of pushing it in without the risk of damaging anything.)
Glassy is bad; the cross hatch scratches trap oil and lubricate the thing. Do you have some way to measure the cylinder diameter? If it's egg shaped, out of round or tapered beyond the Mfr's recommendations, then it's time to cut.
Dingle ball hone does fine, easy to use. If you have a 100 psi disparity between jugs, it doesn't necessariy mean a bad cylinder, could be a valve leaking, bad guides or poorly assembed the last time.
Dan
Yeah, I'm assuming that you tested compression, found it low on one cylinder, pulled the head for the appropriate work, and have since found the bores to be smooth.
Is this on your GS750? (I'm assuming the Rotax is a single.) What kinda miles?
A good ball-honing will restore the cross hatch.
You're gonna do the base gasket, too, right? Highly recommended.
two stories.
GS750 engine is apart. cylinder 1 & 2 were 125 lbs 3 & 5 were 25 lbs. all gaskets looked good but the bores are like a mirror. 35000 miles
the Rotax is a parallel twin. the primary cyl is measuring low. this has not been pulled apart (yet). approx 7000 miles
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