Topic: CB750 Service Bulletin # 42

[Clutch Cargo]

I was perusing the technical documents on this site last night and I came across CB750 Service Bulletin # 42 – Cylinder Head Gasket Oil Sealing (everyone’s favorite topic)

http://www.sohc4.net/files/750_42.pdf

I was aware of the Knock Pin Seals for newer 750 motors, however I was not aware of the recommendation to use eight # 12115-300-305 o-rings as pictured in the TSB.

I wish that I would have seen this bulletin last winter when I had my K1 motor in pieces.

Is this fix still recommended -  ( I could not find 12115-300-305 listed at any of the Honda OEM parts sites)

Is there another o-ring part number that is recommended today?

Mike

[Sweep]

I was just through this confusion when building my '76 750 and I used knock pins and seals.

Just out of curiousity, does your k1 leak oil?

[Clutch Cargo]

 
Minor leaking - but very aggravating as I installed HD studs and Honda OEM O-rings.

[bryanj]

I think you will find that what happened here is Honda "upgraded" the head gasket so that only the one for the "knock pins" and tube seals was available and this has bigger holes arround the studs in these places so "O" rings were recommended for the studs when the block was not couterbored for the knock pins.
If you had an aftermarket set the chances are that if it was labelled up for the early motors the head gasket would not have had big holes to take the "O" rings

[bryanj]

Had a look round for that part, if you go to a dealer with a proper price list that number may show the "O" ring size even though its discontinued, or measure the hole size in the gasket and the thickness plus a bit

[754]

Be careful (on any O rings)what you choose if you are not sure, just a bit bigger will squish down, may not work. A local rider found this out the hard way, destroyed his can and bearings.