Britbike forum
Posted By: MarcB Head gasket leaking - opinions needed - 06/17/20 1:15 pm
I've got a combination of issues that seems to point to an issue getting the head to seal but want to get some opinions on how to proceed.

History: Bike was buttoned up at the end of last summer. Head got new valves and guides by a known good britbike expert. Head gasket was annealed and installed without o-rings. Engine only ran about 100 miles before the season ended. There was a bit of oil weeping up two head studs (left rear, right front). Plug on the left side looked good, right side seemed to have a bit of oil sheen to it.

Over the winter I re-torqued the head and fixed a few other leaks unrelated to the head gasket. Since then, I've done about 1000 miles and the weeping has got a bit worst on the right front stud. Also, the right silencer has a bit of gummy black gunk at the back, indicating a bit of oil in the exhaust.

I pulled the head and this is what it looks like:

Gasket top
[Linked Image from i.imgur.com]

Gasket bottom
[Linked Image from i.imgur.com]

Barrel surface
[Linked Image from i.imgur.com]

Combustion chamber
[Linked Image from i.imgur.com]

Exhaust port
[Linked Image from i.imgur.com]

The bits that concern me are these:
  • Center bolt hole appears cracked at the very top. How concerning is this? The center bolt doesn't slip in and out of its hole in the head so how well it's torquing down is a bit suspect.
  • copper gasket is discolored at the front of the left hole. there is the start of a carbon ridge at the top of the cylinder in that same spot
  • combustion chamber actually has more carbon built up on the left cylinder than the right one. right one is pretty clean
  • while the rear of the exhaust has oily stickiness, the exhaust port is dry (dusty gray rather than gummy black)


I have not pulled valves out but can if getting a better view at the intake tracts is worthwhile.

I don't feel great about reusing this gasket considering the multiple leak points. Are any of the pictures or points above indicative of anything besides the gasket? Any other pictures that would be useful to get posted? Thoughts?
That gasket looks like it is for a bigger bore cylinder.
Posted By: MarcB Re: Head gasket leaking - opinions needed - 06/17/20 8:25 pm
Might just be the angle... you can see from the cylinders and head exactly where the fire ring (if you can call it that) sits, and it's right at the edge of the combustion chamber.
Looks like a 750 gasket. Use the 650type it has more area and will fill the joint properly.
Posted By: MarcB Re: Head gasket leaking - opinions needed - 06/18/20 12:35 am
Nope, unless the 750 has same size pistons, it's still 650
[Linked Image from i.imgur.com]
Posted By: Mark Z Re: Head gasket leaking - opinions needed - 06/18/20 12:59 am
I believe the 750 does have the same bore size as the 650, but I don't understand Andy's and Gavin's comments; it looks like your gasket completely covers the mating surface, so being any larger would accomplish nothing.

It seems no matter what I do, my A65 will eventually ooze oil up around one or more of the head studs. I haven't yet tried O-rings on the oil drain holes, but I have to re-do the head gasket every couple of riding seasons.

I guess there could be an unevenness to one of the mating surfaces; I've resurfaced the head on emery cloth, but I've never done anything to check or true the cylinder barrel.
"Fire ring"? is it a laminated/composite gasket?
If it is chuck it in the bin, it is a use once only gasket. I could never get one to seal on my A10, got a solid copper and never a problem again
"Fire ring"? is it a laminated/composite gasket?
If it is chuck it in the bin, it is a use once only gasket. I could never get one to seal on my A10, got a solid copper and never a problem again
Bin that gasket . Get a solid copper one and anneal . Heat it up proper hot and let it cool , then clean it with scotch brite
Glue some 180 grade abrasive paper to a flat surface , plate glass etc
Swivel the head on the paper to ensure it’s completely flat
Clean all mating surfaces with acetone
Coat both sides of gasket with copper gasket spray
Torque all inner nuts down to 20 lb/ft
Torque again to 25 lb/ft , outer nuts the same
Leave overnight
Torque again to 30 lb/ft only inners
Leave outer nuts at 25lb/foot
Weld a socket to a spanner for outer and torque at 90 degrees to the spanner
After running hot leave to cool and re-torque rear pushrod tunnel nuts x 2 to 30 lbs/ft
Should be ok
After issues with leaks at the back and by the oil drain , this worked for me
Posted By: NickL Re: Head gasket leaking - opinions needed - 06/20/20 11:23 pm
After a close look at the barrel shot, it appears to have liners fitted yes?
I was trying to work out why you have a ring on the head gasket face.
It looks like the left liner has moved down a few thou, that will make the gasket blow, also
the liners appear quite thick, more like car liners than the types i've used.
If the liners are moving it can be a pain to stop them, there are a few dodges but i would
look for another set of barrels first off.
Posted By: MarcB Re: Head gasket leaking - opinions needed - 06/21/20 12:21 am
Thanks

Picked up new gasket, annealed it, cleaned it up and sprayed it with copper coat. Installed it with o-rings over the drain holes, then torqued and retorqued for good measure. Will see how it holds.

Put new plugs in and cleaned the exhaust tips of the sticky gunk.
Ah , liners, thats why there was that mark, the fire ring threw me, I didnt know you could anneal composite gaskets. I thought the gasket was for a big bore, looking at the pics it clearly isnt.
Could you not skim the tops of the barrels and use a thicker solid copper gasket
The idea being to buy the gasket and then take off the material to suit thus preserving the comp ratio .
I’m aware the barrels should realistically protrude a few microns but it’s cheaper than new ones
Posted By: MarcB Re: Head gasket leaking - opinions needed - 06/21/20 12:35 pm
Originally Posted by NickL
After a close look at the barrel shot, it appears to have liners fitted yes?
I was trying to work out why you have a ring on the head gasket face.
It looks like the left liner has moved down a few thou, that will make the gasket blow, also
the liners appear quite thick, more like car liners than the types i've used.
If the liners are moving it can be a pain to stop them, there are a few dodges but i would
look for another set of barrels first off.
Sorry, I missed this response earlier.

You may be right about the liner shifting, although I'm not sure whether it's "shifted" or "mis-installed". The reason I say that is that I've been chasing oil burning issues on this bike since having it lined, which is why it's already 20 over (couldn't get the rings to bed in at standard size).

Unfortunately, it's all buttoned up now but so I can't check the surface with a straightedge to see whether it's actually low or if it's just an issue with the lighting.
Posted By: Allan G Re: Head gasket leaking - opinions needed - 06/21/20 3:55 pm
There’s some places that are good at sleeving barrels and some that manage to screw it up. Seems like you have the latter. I’ve known barrels be sleeved and You wouldn’t get that step on the head gasket surface.
Liners should have a step at the top to stop them moving. Rover (Buick based) V8s had plain liners, it was their achilles heel
Posted By: Allan G Re: Head gasket leaking - opinions needed - 06/21/20 9:44 pm
Do you mean a top hat Andy? I’ve seen liners which are flush at the gasket surface and don’t budge, I’ve also known of some places doing liners and they only hold to the bore in 2 points
Posted By: NickL Re: Head gasket leaking - opinions needed - 06/21/20 9:53 pm
The problem with fitting liners that thick is that the barrel is very thin near the base flange and is very likely
to crack. Effectively the barrels have been bored about +0.200 Those liners are way too thick in my opinion.
You could have a few thou skimmed off the surface to true it up but if the liners are moving then more radical
measures need to be taken like drilling the base flange and putting pins or bolts in. Through bolting should also
be thought about as you've very little metal left on the barrel for strength.
I've had several sets of liners done over the years and never used stepped ones, if they are fitted properly
plain liners in iron barrels are fine. In alley barrels a step is nicer but that's if there is room.

Easier to find another set, they are cheap enough.
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