Topic: cylinder wear help? 78 CB750 SS F -- HOW FAR TO HONE?

[filipo]

Gents,

I've looked through the forums and seen a lot of pictures, but I still can't understand *where* the wear on my cylinders ends/begins. In Hondaman's book, he writes that you should be able to see where the top ring of a piston stopped its travel within each cylinder. But I see a couple of types of wear in mine. See this photo:



There is a very clear delineation between the darker brown/rust area at the top of the cylinder, about 1/4" thick, and the lighter gray/black area below it. There are also a couple of fainter, whitish lines in a doubled pattern, well within the gray/black area.

Should I be looking at the red/brown area as the line where the top ring stopped its travel?
Or is this where the crown of the piston itself stopped its travel? and the fainter, whitish lines are where the rings stopped their travel?

Or should I not worry about it, and hone the whole cylinder so it's all one sheen? 

I'm also posting this in my (too-long dormant) build thread; thanks for any and all answers.

[scottly]

Can you feel the edge of the area 1/4" down into the bore with your fingernail?

[filipo]

Uh oh, I knew this one was coming...

On #1 & #2, not at all.
On #4, verrrrrrrrrrrrry slightly.
On #3, yep, definitely. Not like I get my nail caught up in there, but I can feel it for sure.

[filipo]

Ok, now the suspense is killing me...
What does being able to feel the edge mean?

[scottly]

That edge is formed by the upper travel of the piston rings, at the top of the wear area. It is referred to as the ridge. If you have cleaned any built up carbon, and can still feel the ridge on #3, you should probably have the bore just below it measured to find out how worn it is. Might not be a problem...

[filipo]

Is it worth honing first? Or should I measure first?

[scottly]

Always measure first. BTW, a dingle ball hone will follow the worn surface somewhat, and should only be used to lightly remove the glaze on the bore, not to reshape it.

[PeWe]

Time for an upgrade? 836cc + a mild cam as Webcam 41...

[cgswss]

You can buy a complete overbore kit on e-bay for like $110.  That is almost like getting the pistons free  (by the time you buy gaskets and rings for your motor)  Why not just start with everything nice and fresh and get a little performance boost as well?

http://stores.ebay.com/cruzinimage-co1484/HONDA-/_i.html?rt=nc&_fsub=1242597011&_sid=526414511&_trksid=p4634.c0.m14.l1581&_pgn=8

These people ship from Japan, but the service is very fast.  I got my parts in just about a week.

If you don't do the over bore thing, I would "break the ridge" first Then hone each cylinder with a nice cross hatch pattern.  Of course you will be installing new rings.

The time and cost of pulling the jugs on a bike is enough that you don't want to cheap out on rings and honing and make the whole job worthless.  Of course you should do a leak down test on your valves while the head is off-just in case you need to do the valves while you have it apart.

[Pat_at_APE]

Those e-bay pistons would not work well on the CB750F 77-78 models because the combustion chamber design is different from the K models.  You would loose compression (and power).  You need pistons designed for your specific model. 

Regards,
Pat at APE

[filipo]

Hey now, calm down there, fellas! I don't want to get huge unless I have to. I just want to know if I'm looking at the right place. Is the ridge where the piston has stopped its upward travel?

Scottly: I have a 3-stone hone, not the dingleberry kind. I imagine this means a more even hone (i.e. less chance of the stones following the wear), am I correct?

cgswss: Agreed about pulling the engine. I have a new set of rings recommended to me by a fellow 750F guy. I did a leak test on the valves, they were fine -- filthy, but fine.
What do you mean by "break the ridge"?

Pat: I checked the link, and no pistons for 78 F anyway. Thanks for the tip! (Off to gawk at APE website now...)

Thanks all!

[SOHC Digger]

If APE doesn't have 77-78 750F specific ones, CycleX does:
http://cyclexchange.net/Honda%20cb750%20%28%2077-78%20%29%20%20F%202%20Section%

836cc
20Page.htm

970cc!!!

[filipo]

Gaah. OK, started honing with a 3-stone hone, honed for probably 6 minutes total on cylinder #1 (moved the hone up and down, went slow/medium speed, stopped every 60-75 seconds to clean the stones with kerosene).

I can see the cross-hatch marks show up very nicely, and the big brown ring at the top of the piston's stroke has disappeared -- but I can still see a few faint leftovers the upper wear marks. I can't feel anything, my fingernail doesn't catch, it's very smooth (to my scaly fingers, anyway) -- so can I stop? Or should I continue until there are NO wear marks whatsoever?

Thanks, any quick replies much appreciated, since I've finally gotten around to this but don't have much time (as always)...

Phil

[CB750F2]

Now that you have honed the cylinders it would be a good idea to measure the piston to bore clearance. I think that the service limit is 0.0024" but check the manual. Pat

[wrenchmuch]

You should hone till you can't see marks left by the rings. When you're done that's when you can measure bore diameter to see if you're still within spec for your pistons.

[tlbranth]

The ridge at the top of the cylinder is the heaviest wear point. Here's how I'd do it: rent a ridge-reamer. This tool will, if used correctly, remove the ridge from the top of the cylinder. If you don't remove the ridge, you risk breaking your new top ring. Hone the cylinders. I've always used a homemade hone consisting of a dowel with a slit sawn lengthwise for 2 or 3 inches. I use a piece of emery cloth doubled over in the slit and chuck the dowel in a drill. Been doing it this way for 50 years. Check the ring gap on each ring. If they're OEM they are probably OK but you definitely don't want too small a ring gap. Use 2 ring compressors. Oil the rings. You must remove the ridge! Honing will not do it.

[scottly]

Run the hone with the stone pivot points just below the ridge on the up stroke, and use short strokes, like 1/2". This will focus the cutting on the ridge area. Check often. You are looking for an even cross-hatch. A pic of where you are now might help.

[filipo]

Hurgh. I'm attaching pics, but the bad news up front: Post-hone and after cleaning, it looks like I'm out of spec. I put a .0025" feeler in there with the piston upside-down and the skirt at the bore's opening, per Hondaman, and the feeler slid out with minimal resistance. Maybe I'm wrong, I haven't used a feeler gauge since the 1990s -- the idea is I wouldn't be able to pull the feeler out, right? Or it would be a slight struggle? This one comes out relatively easily. So I think I am out of spec. I am sad.

Some photos, and then some questions.

Where I started


After 1 minute hone

After 6-8 minutes hone


All great, wear marks gone, whoop-de-coo -- because clean as it is I still need to overbore. Which is annoying.

If I overbore, I can go one over stock (i.e. 0.25), correct? And all I'd need is new pistons and rings?
Whereas if I go to, say, an 836 kit, that means all kinds of new gear -- studs, valves, cam, all of it?

Thanks everyone. Little bummed tonight.

[scottly]

How big of a feeler gauge will fit? (I think you were pretty close on the hone job after 1 minute; 6-8 might have been pushing it a bit. )

[filipo]

Just going by what Hondaman says. Also I thought I needed to get rid of all wear marks. Did I not?

At any rate, just to check I popped the same piston into cylinders #2 & #3 -- same basic situation. Might instead take the whole thing to a real machine shop and bag this DIY game.

[scottly]

So what size feeler gauge will fit? Normally, in a proper machine shop, bores are checked with a specialized dial indicator, for diameter, taper, and out of round. Also, the pistons should be measured with a micrometer. The wear of the bore, plus the wear of the pistons affect the clearance. I'm not sure what the service limits are for the bore or the piston off the top of my head.

[filipo]

I've read .0024", from Terry in Oz and others. I got a .0025" in there, stopped trying due to despair.

[scottly]

Good God, you're worried about one ten thousands of an inch? Will a .003" feeler go in freely?

[filipo]

Yeah man! I'm worried about everything, I've never done this before.

The .003 does go into the honed cylinder; however it does not go into #2 & #3. So my guess is I overdid it. Or maybe I did what I was supposed to do (i.e., get rid of all wear marks), but there was no helping it.

Any suggestions?
Thanks.

[scottly]

I believe the .0024 number is referring to new, forged pistons in a fresh bore? According to my old Clymers, the wear limit on the bore is 2.406", and the wear limit on the piston is 2.394", so worst case would be a clearance of .012". (You may want to verify this with the official Honda manual.) What is the largest feeler that will fit? Also, be aware that the thicker the feeler, the harder it is to make it bend to fit the curve of the cylinder. If you're really worried, go ahead and clean up the other cylinders, then have them and the pistons measured with the proper tools.