Topic: CB750 K0 - Stopping head gasket oil leaks

[kaysystems]

Some time ago I posted about the Honda recommended O-rings for the head gasket on the CB750 K0, as outlined in Service Bulletin #42.
Here's a photo of one O-ring in place. (on the closest large stud)

These are 12115-300-305. They are getting difficult to find, but I'm sure they will stop the leaking from the oil drains.

David

[Kevin D]

Kay,
I have TWO new head gaskets, Honda and Vesrah, but neither has an opening to match that oring like your does. Whats your head gasket p/n? I also tried 12020-300-305 just like the service bulletin says but it didn't fly.

Can you tell me a dimension for the oring?

[kaysystems]

I beleive I bought 12251-300-040 from Honda. As far as I am aware all the New genuine Honda gaskets have the large holes. This accomodates all models from 69 onwards, no matter what the seals are. I bought mine from CMSNL.com

However if you get a gasket for a K2 and later it will have the large holes.

Mine actually came as part of a top end gasket set from Honda. It was for a K2. The O-rings for the oil feeds under the cam bearings were the wrong size, but I bought them seperately from Honda.

Need comments from MRieck or HondaMan

David

[MRieck]

I beleive I bought 12251-300-040 from Honda. As far as I am aware all the New genuine Honda gaskets have the large holes. This accomodates all models from 69 onwards, no matter what the seals are. I bought mine from CMSNL.com

However if you get a gasket for a K2 and later it will have the large holes.

Mine actually came as part of a top end gasket set from Honda. It was for a K2. The O-rings for the oil feeds under the cam bearings were the wrong size, but I bought them seperately from Honda.

Need comments from MRieck or HondaMan

David

All the new OEM head gaskets have the larger holes as as I know. I sold about 6 of them on ebay several years ago . The one pictured is a newer Honda graphite head gasket. It has the oblong fire ring etc. The last Vesrah I saw (approximately 1 year ago) had large holes too. A NOS Honda head gasket could have the smaller holes but that would be pretty old. The Cometic replacement gaskets (736, 836 and 915) have small oil return holes. The very early K gaskets has copper orings built into the head gasket around the oil return passages too. Those "orings" David showed are flat faced all around. He was nice enough to give me an extra he had.

[kaysystems]

I'll measure an O-ring tonight & post the size

David

[markb]

I had the same problem with the Vesrah kit and ended up buying a #12251-300-070 head gasket from Honda. That did the trick.

[kaysystems]

Did my best at measuring.

17.4mm outside diameter
1.35mm thickness
these are square section (not round)

David

[Kevin D]

Thanks guys,
 Looks like I'll be getting another HG unless I can find some sort of punch to knock these holes out. I guess thats a hazard of picking your own part numbers. My part number for the Honda head gasket at the bottom is 12251-300-307:

[kaysystems]

The one at the top looks OK from the picture. it has the larger holes.

Make sure you can find the O-rings though, before you go buying another expensive gasket.

David

[bwaller]

You should be able to get viton o-rings from a bearing outlet, although having an original to take for them to measure would be wise.

 I just picked up some to replace the eight on the carb inlet tees, in viton also, after being told these too were no longer available from Honda.

[MRieck]

You should be able to get viton o-rings from a bearing outlet, although having an original to take for them to measure would be wise.

 I just picked up some to replace the eight on the carb inlet tees, in viton also, after being told these too were no longer available from Honda.

The difference is the "orings" Dave is talking about are flat faced. They sit better in that gasket hole. Judging by Dave's measurements I'd say crush on them is about .010 to .012

[Dave Wyatt]

I used the Victor-Reinz head gasket that came in the Versah kit, along with the O-rings.  I didn't worry about the smaller holes in the gasket and just put the damn thing together.  The O-rings will conform to the opening anyway, so it should not be a big deal.  The much higher quality head gasket is what should make the big difference.  The OEM gasket was a POS IMHO.  It should not have been acting like a wick.

So far I have about 300 miles on the bike since the re-gasketing, and no leaks.

[bwaller]

Yea Mike I hear ya concerning those square rings, have a bunch here. I may check out getting replacements from my local joint.