Author Topic: Piston Swap (Read 5601 times)

Sheik Yerbouti · « **on:** October 29, 2009, 11:44:23 AM »

Hey guys.

I'm having hell of a time finding piston rings to fit a '78 CB750K. I was browsing ebay and found several OEM sets of 4 pistons for a reasonable price. I also found the rings for them no problem. They're 1969-1975 pistons. Rings can be had much easier (and cheaper) for these pistons. I just want to make sure there would be no problems with the swap.

Link:

http://cgi.ebay.com/ebaymotors/1969-1975-Honda-CB750-Pistons_W0QQitemZ150383965246QQcmdZViewItemQQptZMotorcycles_Parts_Accessories?hash=item230395343e#ht_500wt_1182

Thanks guys.

moham · « **Reply #1 on:** October 29, 2009, 05:12:43 PM »

I think those pistons might have a shorter crown so you might end up with a lower CR than with the k8 pistons but you could remedy that, if in fact that's the case. Not one hundred percent on my analysis...

Sheik Yerbouti · « **Reply #2 on:** October 29, 2009, 06:32:34 PM »

Thanks. Does anybody else have any ideas before I take the plunge? How could I remedy the lower CR?

faux fiddy · « **Reply #3 on:** October 29, 2009, 11:48:54 PM »

Have you seen this guy's store?

http://myworld.ebay.com/ebaymotors/motorcycle-parts-supply/

I gotta reccomend him.

Sheik Yerbouti · « **Reply #4 on:** October 30, 2009, 07:04:08 AM »

Unfortunately he doesn't have the rings I need.

moham · « **Reply #5 on:** October 30, 2009, 09:12:11 AM »

It might be more work and $ than you want/can do/spend but milling the head a bit would bring the CR back up. I don't know if this would create other issues (cam chain fitment) bu others might have more to say on this. I still think you should be able to find those rings, even in an over bore size (+.25 etc).

Have you tried calling Western Hills Honda or Powers Edge? David Silver? I wouldn't rely on the online parts checker for availability. Calling these guys might prove more fruitful.

I hope I'm not giving you bad info here. Maybe some of the 750 gurus will chime in soon to help get you sorted. Maybe try pm Terry in Aus or Mrieck to help out...Good luck.

faux fiddy · « **Reply #6 on:** October 30, 2009, 11:16:18 AM »

Milling the head might work, and the slight bit taken off probably would not be an issue as you can tighten that amount of slop out.

I'm not a machinist, but I know a very good one and that's for sure what you need.

Milling the head raises issues as the thing not only has to be perfectly flat, but the cam has to be flat and running in a paralell plane. Sometimes that doesn't happen is what I'm told.

Sheik Yerbouti · « **Reply #7 on:** October 30, 2009, 12:02:27 PM »

I was hoping to do this all on the cheap...

Wasn't there somebody who would turn the proper sized ridge in the piston for 25 bucks a pop? That might be the way to go.

moham · « **Reply #8 on:** October 30, 2009, 01:29:04 PM »

Sheik, powers edge motorsports can get them from Honda. Call them, western hills i'm sure could get them as well. I didn't ask price but Honda shows them in stock...

faux fiddy · « **Reply #9 on:** October 31, 2009, 12:16:38 AM »

There are several articles on cutting piston rings.

http://www.google.com/search?q=Cutting+Piston+Rings+&rls=com.microsoft:en-us:IE-SearchBox&ie=UTF-8&oe=UTF-8&sourceid=ie7&rlz=1I7TSHB_en

Sheik Yerbouti · « **Reply #10 on:** October 31, 2009, 08:11:04 AM »

Quote from: poor boi on October 31, 2009, 12:16:38 AM

There are several articles on cutting piston rings.

http://www.google.com/search?q=Cutting+Piston+Rings+&rls=com.microsoft:en-us:IE-SearchBox&ie=UTF-8&oe=UTF-8&sourceid=ie7&rlz=1I7TSHB_en

The thing about the '78 rings is that I think one of them is narrower than pre '76 rings, meaning the grooves in the piston have to be modified.

Jerry Rxman Griffin aka MuthaF'er · « **Reply #11 on:** October 31, 2009, 06:26:30 PM »

I had some 73.75mm 1080 pistons cut for 3 piece oil rings at Total Seal

Ed, at Total Seal also found 3 piece oil rings and compression rings that fit my 900 Henry Abe kit without cutting.

Hastings ID'd the rings for my Yoshi 810 kit.

Yes you can get the rings from Honda, probably at a premium, but by the time you cut your pistons, pay shipping 2 ways AND buy rings you're probably out more. Call Ed at Total Seal and see what he can do. I think my 4 sets of oil rings cost me $35 total.

moham · « **Reply #12 on:** October 31, 2009, 07:04:16 PM »

$33.19/set from www.powersedge.com

754 · « **Reply #13 on:** October 31, 2009, 07:21:00 PM »

maybe we shoulda warned you...
oh yeah, we did....

>>>>>>>>>>>>>>>>>>> running >>>>>>>>>>>>

Sheik Yerbouti · « **Reply #14 on:** October 31, 2009, 08:59:37 PM »

thanks

Industrial Cafe · « **Reply #15 on:** October 31, 2009, 09:10:16 PM »

heh, I just realized your screen name is "shake your booty" fu*kin' awesome man! hahaha

Sheik Yerbouti · « **Reply #16 on:** November 01, 2009, 08:39:37 AM »

Real quick.

Do 77 and 78 supersport rings work on the K?

Sheik Yerbouti · « **Reply #17 on:** November 01, 2009, 09:42:11 AM »

Well, just bought these.

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=360199870477#ht_566wt_940

I figure even if they aren't right I could sell them and make my money back.

moham · « **Reply #18 on:** November 01, 2009, 02:16:07 PM »

Different part nos.

Sheik Yerbouti · « **Reply #19 on:** November 01, 2009, 02:31:39 PM »

I noticed that. But I searched these forums and read that Honda made the part numbers different but they're the same rings. Figured they were worth a try.

Industrial Cafe · « **Reply #20 on:** November 01, 2009, 02:48:43 PM »

try it and report your results

Sheik Yerbouti · « **Reply #21 on:** November 01, 2009, 02:52:41 PM »

I figured for 15 dollars a set I couldn't go wrong. I bought 2 sets, I'll take some measurements and compare them to a set for 78K (if anybody has a set and wants to report measurements that'd be great).

If not I'm sure somebody else could use them.

moham · « **Reply #22 on:** November 01, 2009, 03:33:24 PM »

Quote from: Sheik Yerbouti on November 01, 2009, 02:31:39 PM

I noticed that. But I searched these forums and read that Honda made the part numbers different but they're the same rings. Figured they were worth a try.

Well, when you look at the different rings offered for the K8, you will notice different part nos. for three different manufacturers of rings for the K8. But when you compare the three different part nos. for the K8 to the nos. given for the F model, they still don't match up. I'm not saying they won't work, because I don't know if they will, just that this might explain different part nos. for the same size rings.

My question is this: if the proper rings are readily available for the K8, why mess with these other rings which may or may not work? Is it a cost issue? Just curious...

Industrial Cafe · « **Reply #23 on:** November 01, 2009, 03:54:44 PM »

Quote from: Sheik Yerbouti on November 01, 2009, 02:31:39 PM

I noticed that. But I searched these forums and read that Honda made the part numbers different but they're the same rings. Figured they were worth a try.

that's why God made files.

Sheik Yerbouti · « **Reply #24 on:** November 01, 2009, 04:16:59 PM »

Quote from: moham on November 01, 2009, 03:33:24 PM

Quote from: Sheik Yerbouti on November 01, 2009, 02:31:39 PM
I noticed that. But I searched these forums and read that Honda made the part numbers different but they're the same rings. Figured they were worth a try.

Well, when you look at the different rings offered for the K8, you will notice different part nos. for three different manufacturers of rings for the K8. But when you compare the three different part nos. for the K8 to the nos. given for the F model, they still don't match up. I'm not saying they won't work, because I don't know if they will, just that this might explain different part nos. for the same size rings.

My question is this: if the proper rings are readily available for the K8, why mess with these other rings which may or may not work? Is it a cost issue? Just curious...

Entirely a cost issue. I'm rebuilding this on a student's budget.

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Author Topic: Piston Swap (Read 5601 times)