Cylinder replacement (what else has to happen?)

[vdavidoff]

I don't believe I need to replace any of my cylinders, but one is damaged externally and I'd like to understand what I'd be getting into, mostly in terms of dollars, to fix it.

The bolt holes for the exhaust mount on the right cylinder are completely stripped out. One of them is actually totally gone, and the other is almost as bad. There's a bolt with a nut on the end run through the stripped hole that still has a full diameter of metal, but it has been drilled/stripped out so badly that the outside wall of the hole is extremely thin.

If I found a donor cylinder what else needs to happen? I am getting the impression that I'd need to do something with the piston to help ensure it fits together with the cylinder properly, and that I couldn't just throw the new cylinder on. If that's accurate, what will it take? I am hoping that maybe we're just talking about replacing piston rings, but am feeling like you're gonna tell me that won't cut it.

As always I searched the forum first but couldn't find anything specific in this regard. Apologies if it's in there.

Thanks for your advice.
Andy

[H2RICK]

Andy, ANY replacement cylinder will have to:

AA) be checked for correct bore size (to match your existing piston, assuming you're going to keep it). Assuming the cylinder turns out to be the correct bore size, then.....

BB) it needs to be checked as to whether or not it's still within tolerance to use your existing piston, and then.....

BB) the cylinder will have to checked with a bore gauge for ovality, from side to side and front to rear, and....

CC) checked for taper, from top to bottom, and....

DD) a check of its deck height to make sure that no one has machined either the top or the bottom to change port timing or compression ratio.

Assuming ALL these measurements are within spec, then a new set of rings in the correct size, a light hone to remove any glazing on the cylinder walls, a cleanup of the edges of all the ports to prevent ring breakage and a final thorough bath in clean solvent should finish everything off nicely.

[tz375]

Alternatively, pull your block - no mean feat in and of itself, get the holes cleaned out and weeded up and drill and tap back to size. Probably $40 plus some time plus the same gaskets.

[vdavidoff]

H2RICK - Thanks for so much detail! I am sure that's going to be very helpful at some point.
tz350 - Thanks for the reply but I don't exactly follow what you're suggesting. It sounds like you're talking about having the bolt holes in the cylinders retapped, but I don't follow the comment about the block (unless you're just saying pull the block out of the frame to get the cylinders off?)

Thanks for the replies.
Andy

[Coyote]

H2RICK, tz350 How do you know what bike it is?? I can't believe how many guys post questions and never bother to mention WHAT they are working on.

[H2RICK]

Chris, regardless of make/model/year my recommendations would be the same....except the remarks/instructions about "ports" and "port timing" could be left out if Andy was referring to a four stroke cylinder.
The only thing I didn't mention was a warning about score marks or corrosion pits and the cylinder probably needing a rebore depending on the presence/severity of either of those conditions.

[tz375]

Andy,

What I was suggesting was removing the existing block to repair the threads. It may be possible to clean out the damage , weld them up and re-tap the threads if they are really bad or repair the threads with helicoil or timeset.

With a GT750 removing the block can be a PIA but your prior posts were about a 550 so I am assuming that's what we are talking about. To remove the right cylinder, I don't believe the motor has to come out of the frame, but I could be wrong. Carbs exhaust and head have to come off first.

Coyote and H2Rick both have 550s and can give you more detailed instructions if you need them.

[Coyote]

vdavidoff, I have a spare right cylinder. Stock bore in good shape and the exhaust threads are good. I does have a couple of dented fins form some PO having the kick starter lever out of whack and allowing it to slam against the fins. If we are talking about a GT550, it's yours for the price of postage.
You can pull the cylinder with the motor in place.

[vdavidoff]

Thanks all. Sorry for not including the bike details. It is the '74 GT550 I have been asking other questions about. It didn't occur to me that the answer to this question might be bike specific, but that was a mistake.

tz375 - The word "weeded" in your first post is what confused me. Now that I see you mean I might be able to have the holes "welded" and tapped out again, I think I'm going to consider that before I consider buying new cylinders. I didn't realize this could be done.

Coyote - Thanks for the offer on the cylinder. If I decide to go that route I'll be in touch about it.

Thanks again, all.
Andy

[Coyote]

Here's a pic of the damage on the fins. A little work with a file and it wouldn't look near as bad. Want a pic of the exhaust threads and bore?

[tz375]

At that price, I'd grab Coyote (Chris's) generous offer.

As for my typing, well I guess I'll never make it as a typist or admin assistant. My writing is bad enough for me to be a doctor though I must be spending too much time in the garden doing yard work I guess.

[Cliff]

Here's another option if you want to go this route. Drill and tap the two holes to 1/4 NPT. Put in two 1/4 inch pipe brass countersunk plugs, making sure they are in flush with the outside, and the then drill and tap the plugs to use the original 8mm bolts. Pipe thread is tapered so only run the tap in about two third of ot the way. When the plug is installed, because it's a tapered thread, it'll start in very loose but really tighten up the farther you put in!

[vdavidoff]

Here's a picture of how bad it is. I don't think this can be retapped without doing something like tz375 suggested and having the hole welded up first. Ya think?

[H2RICK]

A guy that knows what's he's doing can weld that back up to the point that a new 8mm hole could be drilled and tapped into the new weld.
The "fun" part will be the actual fixturing to get the welded-up portion correctly situated for drilling and tapping ACCURATELY. This would NOT be work to be attempted with with a hand-held drill, IMO. After the welding, you're only going to get one shot at getting it right.
Unfortunately it doesn't look like there's enough metal there to do what Cliff suggests.....although Cliff's idea is a great one which will go into my memory banks for future use. Thanks, Cliff.
Andy, you just may end up going with Coyote's offer of the cylinder as the cheapest/easiest fix. A guy that does aluminum welding can easily build up those chewed-up fins and with some Dremel and hand file work on your part and some careful glass bead blasting in that area no one will ever know a repair has been done.....or just clean things up with a file so it's not so jagged and use the cylinder as-is.