Topic: The K8 Piston Ring Debate

[Sheik Yerbouti]

Well, I've been doing some research on replacing the rings on 77-78 cb750ks and it seems there is a lot of confusion about them. Some people say the rings are different/thinner/something or other. But I just contacted Keith Jones at Total Seal. He sent me this:

Jon,
I do not believe they are different. I'm pretty sure they use the same ring pack According to my info they are 61mm bore std and use a 1.5 1.5 2.8 ring pack. I had someone send me a piston that insisted had a different ring pack but it did not. I don't think he had the proper equipment to measure the grooves. He sent it in to be re-grooved but it did not require this. Check your groove sizes and let me know.

So does that mean 71-76 rings will fit just fine in a 78? Does anybody have any first hand experience? That way I could get a set of rings much cheaper.

I appreciate it.

[markjenn]

I always understood the 77-78 F2 rings were different, not the K's.

- Mark

[eldar]

The F pistons are different as the combustion chambers are different. I always thought the K bikes were all the same, however the 77/78 k engine is derived from the 75/76 F engine.

[Sheik Yerbouti]

Hm, I guess I'll have to take a look when I open up the top end and send him some measurements.

I was getting some really high wet compression readings, as high as 206 psi. I wonder if the previous owner threw in some high compression pistons or something?

[martino1972]

Hm, I guess I'll have to take a look when I open up the top end and send him some measurements.

I was getting some really high wet compression readings, as high as 206 psi. I wonder if the previous owner threw in some high compression pistons or something?

does this mean you bought the bike??

[BobbyR]

Hm, I guess I'll have to take a look when I open up the top end and send him some measurements.

I was getting some really high wet compression readings, as high as 206 psi. I wonder if the previous owner threw in some high compression pistons or something?

If you have high compression, why change the rings? Your screen name was a pretty good Zappa album.

[SD750F]

As I understand and have direct experience the F2 and F3 has a thinner top ring. The rest are standard as the K series. I had my F3 pistons re-grooved so I can use standard ring sets that you can still buy.

Scott

[eurban]

I have read a bit of your other posts . . . I would suggest that you actually attempt to get your new bike running before you get all focused on engine teardown and  piston rings etc, etc.  It was actually mentioned in your other thread that bikes that have been sitting a long time are not likely to yield good compression readings even though they may be fine once they have been put back in use for a few miles.  Also, compression gauges, adapters and technique can make a good bit of difference in the numbers you will read so it is possible that your high reading is a not a reliable result.  The bike has many systems which will need some attention if neglected  so it would be wise to go through a complete tune up procedure and likely a carb cleaning.  After this work, you may just find that the bike will start up and run just fine without the need for an engine rebuild. 

[Sheik Yerbouti]

Just planning for the future you know. Getting all the research out of the way beforehand so I know exactly what I'll need.

[eurban]

Sometimes too much worrying will paralyze you . . .If you haven't already, buy the bike for as little money (from what you have said, doesn't seen like it would be any more than $400) as you can and dive into it. . . .  The 77/78K bikes have a bit higher compression ratio than the earlier Ks and the pistons are certainly partially (likely totally) responsible for the bump.  I don't know if the rings are different but the pistons certainly are.  Go to on online microfiche like www.powersportspro.com and compare part numbers with earlier K bikes if you like.  If in the end, your pistons really need new rings, consider budgeting for a big bore kit instead. 

[754]

Maybe it just has a tablespoon of oil in the cylinder..

[Soos]

I put high domed F pistons in my 650 as an overbore.

I could not (for a reasonable price) get ahold of "F" piston rings, and after asking a few questions here determined that the K's would HOPEFULLY suffice at least for the 2 top rings.

The pistons I bought (used off e-bay stocker 61mm F pistons) came with the original 3 piece oil rings.
Didn't matter on the 1 piece oil control ring anyway... as I remember it was too thick to fit in the F pistons, and would have needed machining the pistons to fit properly.

Other than that, the top 2 rings were either the same, or close enough i didn't care.
And the rings I purchased were for K pistons.
It runs to this day about 2000 miles later.


l8r

[eurban]

According to the parts fiche the 77 and 78 Ks use a different variety of part numbers for their piston rings than the earlier K's (all the 76 and earlier Ks and the 76 and earlier Fs list the same part numbers for piston rings). For the 77/78ks there seems to be three ring manufacturers and depending on what engine number the bike has there is a corresponding ring set listed.  I cannot say if there really is a difference in the piston ring grooving or not however.  Powersports pro does NOT list the 77/78k ring sets as "unavailable" which they usually do if the they can't get a particular product.  Parts are listed as $33usd per ring set. . . . .

[Clyde]

The F1 changed to thin rings at engine number 2551568 (late in the run) and these rings were also used in the K7/K8.
I believe the F2/K7/K8 rings are the same thickness, but there were several part numbers, presumably as a result of different suppliers