Big bore head gasket

GregMcN · Post by **GregMcN** » Wed Sep 27, 2017 3:47 pm

When I purchased my CB77 I was told it had a 350 conversion but given that not everything else has been as claimed I wanted to check for myself.
The good news is that it does seem to have one, I have measured the bore at about 63.75mm.
The problem I do have it that it seems to have a standard CB77 head gasket, measures about 62mm.
So question is where can you get a suitable head gasket, are there any from other models that are suitable or anyone sell specialised product?

Thanks Greg

border0_3 · Post by **border0_3** » Wed Sep 27, 2017 4:18 pm

Hi Greg,

If the bore is 63.75mm maybe a 3mm oversize head gasket will suit.

Normal bores for 350 kit I would have thought would be 64 or 65mm.

This company deals with big bore kits and may be able to help further.

https://bore-tech.com/categories/honda-305

Best regards

Richard

GregMcN · Post by **GregMcN** » Thu Sep 28, 2017 7:05 am

Thanks Richard

I have no idea how the oversize was achieved, whether it is a kit, new sleeve or just over bored though looks to be plenty of metal. Did not know they went to 3mm oversize and have not seen any gaskets. I assume I could use a standard gasket but would use a lot of the benefit from the oversize?
Boretech seem to do a 66mm but I also see a 64mm copper gasket advertised on ebay but not sure of any problems / benefits of copper gaskets.

Greg

GORDON BROWN · Post by **GORDON BROWN** » Thu Sep 28, 2017 2:39 pm

Hello Greg

If the liners are still fairly thick then they are most likely replacements as the STD ones are very thin at 63/64mm bores. ( would you believe the liners in the four cylinders in the photo are all different ), From the photos it looks like the pistons are modified ones from another model, Kawasaki or Honda perhaps ? or even a period aftermarket make.
If you could post some photos someone might have the answer.
Regarding the head gasket, a copper one can work ( if the surfaces are good ) and you might need a bit of sealant.
I am sure I saw someone selling 64mm gaskets in the UK so it is worth checking.
If the pistons do not come too far up the bore and the combustion chamber bore is STD then you might get away with a STD one but even if the pistons are modified ( stepped ) for this its far from ideal.

Gordon
SAFOJ

GregMcN · Post by **GregMcN** » Thu Sep 28, 2017 4:26 pm

Hi Gordon

Not found anything in the UK yet.
I did read a post which I think was saying that they cut the centre rings out of a cb350 gasket + 1/4 inch and used these with a standard gasket.
Surprised this worked but I guess if you cut accurately the seal is just around the piston and the rest is just you to allow torquing down the head evenly?

Greg

LOUD MOUSE · Post by **LOUD MOUSE** » Thu Sep 28, 2017 6:48 pm

May work but chancy at best. ...................lm

GregMcN wrote:Hi Gordon

Not found anything in the UK yet.
I did read a post which I think was saying that they cut the centre rings out of a cb350 gasket + 1/4 inch and used these with a standard gasket.
Surprised this worked but I guess if you cut accurately the seal is just around the piston and the rest is just you to allow torquing down the head evenly?

Greg

G-Man · Post by **G-Man** » Fri Sep 29, 2017 3:28 am

These are the boys......

https://bore-tech.com/product/head-gask ... g-bore-kit

G

GregMcN wrote:Hi Gordon

Not found anything in the UK yet.
I did read a post which I think was saying that they cut the centre rings out of a cb350 gasket + 1/4 inch and used these with a standard gasket.
Surprised this worked but I guess if you cut accurately the seal is just around the piston and the rest is just you to allow torquing down the head evenly?

Greg