Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: captaink on May 30, 2017, 04:47:18 PM
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Hi, Any thoughts on the following appreciated.
I'm in the process of building a 1977 k7 engine and its now got a Wiseco 836cc big bore kit fitted.
The Cometic head gasket that came with the Wiseco kit only has two larger (17mm ) hole cut outs for the studs with O rings the rest are just stud clearance size.
My cylinder has the 2 plus 8 more for the knock pins and seals, 10 X 17MM holes are required in total.
The Wiseco kit supplier says there's only ever been this gasket which is wrong for me.
The standard gasket kit from Silvers has all 10 larger holes but of course the cylinder bore size is standard 750cc and too small.
I have seen gaskets on USA sights but wondered if anyone has had this issue and know a UK supplier ?
Regards Keith
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I had the same issue recently.
I ended up drilling the clearance holes out very carefully as whoever had bulit the engine before me had fitted as standard and squashed all the rubbers underneath the gasket. no wonder it leaked before!
To drill out get 2 plates of aluminum made up with a 17mm hole drilled through and bolted together at both ends. place gasket in between line up the hole in the gasket you need to enlarge and drill through.
Remove any burrs from around the hole and hey presto. sorted.
Just need a bit of bravado and belief to drill into a £50 head gasket!
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Enquiries about the head gasket are on going but it appears I can get the correct gasket from japspares in Australia, its a Forsetti gasket.
836cc big bore kit 65mm bore with 10 in total larger 17mm clearance holes
Anyone know anywhere nearing than the opposite side of the planet ?
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Thinking on this subject What about this as an option to solve the problem....Any thoughts ???
The 8 larger holes in the cylinder barrel as fitted with later engines are an upgrade to early engines and require knock pins ( sleeves ) and a rubber seal.
Why cant I make 8 small spacers that accurately fit in the larger holes, flush with the head thereby reverting back to the earlier models ?
Does this make sense as although an upgrade from about 1975 there's a lot of engines out there pre 1975.
I could then use the Cometic gasket with the smaller holes.
Thoughts ?
Regards Keith
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It's relatively easy to make a basic punch and die set to re-cut the gasket holes to the right size which would possibly be easier.
We used to make them specifically to cut gaskets as apprentices, if you want more info it may be worth a chat to explain as it could be quicker and more consice than me typing it.
I'll pm you my number if you want it.
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Hi, I did order a 17 punch and it did work ( I hope )
I did this for my other cb750 I'm building but the 17mm holes are so close to the main fire ring it's worrying.
The Cometic gaskets as most contain metal and the punch soon loses sharpness.
Lets say it wasn't as good as I wanted but we'll see.
I can get a Forsetti gasket from Australia.
Options many !
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We'd make them out of two 1/2 inch steel plates that where dowelled for location then bored and reem the punch hole to desired size through both plates, then make a punch out of silver steel with concave curve to the face. This was heat treated to harden the cutting edge and hand ground (sounds like a waitrose food advert) to get the edge sharp.
It would cut gasket material and metal shim of copper, brass and steel with a very clean unburred edge and no distortion. The work material was clamped in between the die plates and with a hefty wallop, the punch would got straight through.
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I foresee no engineering problem with small spacers, except the original oil leaks that the seals were supposed to cure, as long as the adaptors are sealed in the holes BUT be careful with which cylinders you use as the later ones (F2 I think) had a different oil return path
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I use the supplied gasket with the 836 kits from weiseco, they all have the small hole which is a pain. I machine up a punch to recut the holes in a fly press.
works ok, but just time consuming.
Andy
www.good-bits.co.uk