twin head gaskets

[Groily]

As nothing ever goes wrong with a twin in the engine department . . . hmmm . . . .was surprised yesterday, having routinely replaced the exhaust valves after a mere half-lifetime (using rubber sucker, of course, to grind them in), to see that on start-up one head gasket wasn't gasketting. As in, streams of oil coming out horizontally from the passages for the rocker shafts, accompanied by noises more redolent of steam than internal combustion.
First thought was that a glass of the proverbial and early-onset dementia had got in the way of the old judgment, so took it apart . . . nothing obviously amiss and certainly no prob with the alignment of the inlet manifold etc etc which is always fun. So, for want of a better plan, put it together again to see (using same now-less-new gasket on that side)whether it was it or me that was the clown. With, predictably, the same unhelpful result.

Then, feeling confident it wasn't mis-assembly, took it apart again for a serious look . . . and it turns out that, comparing the brand new gasket from my box of such things with the old one that was in there before, there's a difference in thickness of best part of half a millimetre. Which difference is enough to ensure that the spigot on the cylinder fills the annular recess in the underside of the head before any serious pressure can be brought to bear on the thinner gasket, with the obvious consequence.
Does anyone know what the correct thickness of a 500cc twin head gasket ought to be? Anorak Q, I know, but I'm intrigued now. I don't want to have to mill off a bit of the spigot on the cylinder, I don't think the sleeve - if there is a sleeve in these things - has shifted mysteriously in an upward direction . . . but I do want a proper fit and a gasket thick enough to ensure it . . Grateful for any thoughts. Groily

[Biscuit]

Can't help with the gasket thickness, but the depth of spigot is 0.112 to 0.120, three barrels were measured. I would think it would be unlikely that, if sleeved, the sleeve would replace the spigot.

[wilko]

Maybe the spigot height changed post'56 or the heads have been machined and thicker gaskets used.But you say you've had it apart before? Dugless

[Don Madden]

I measured 0.075in average for several new, unused head G12 gaskets from JSL. I assume the 500cc is the same.

Be sure wire mesh is imbedded inside. Some of the mesh is non-magnetic but can be detected by scraping the edge of the gasket with a finger, often by punturing same. Better is to us a knife & bright light which will reflect of the metal.

Tighten per the advice given previously here, but ignore mine.

Cheers, Don.

[Biscuit]

I don't REALLY think this is the answer, but I fitted new gaskets some time ago and the straight edge of the gasket, ie the edge facing the adjacent cylinder, overlapped onto its companion. This gave a double thickness at this point, not under the head admitted, but the gasket was bent up where they overlapped. I trimmed possibly 1/16" off of each gasket.

[Groily]

Thanks folks. I have had the overlap problem you mention before Biscuit, but not this time. And agree 100% re sleeve/spigot height etc. Spot on. The actual thicknesses, as miked, are, new gaskets 75 thou (tallies with your thoughts Don), old, many-years-squashed ones, 93 and 92. That 17-18 thou seems critical on my lh cylinder at least! Old gasket back in is better, but hardly a great plan, I'm not feeling good about it, and I've got a barrel base leak now that side from having the head off 3 times in 2 days and wiggling things . . . what we need is (maybe?) nice anneal-able copper, of a decent thickness. Also, for what it's worth, remarkable loss of torque setting in a handful of miles . Set at 18, down to 12 in 5 miles on the un-re-disturbed rh side. I've noticed this before with modern replacement head gaskets for my tatty old Riley 9 - it usually takes 3 or 4 check tightens to get it to stay at the right setting (iron head so a lot tighter). Luckily, an AMC twin is a lot easier to do. And yes, Wilko, this motor has been apart a fair few times over the years for one reason or another - and although like everyone else's it has the odd niggle, I have always regarded it as my best and favourite bike, though tatty too, largely because if one does what Mr Neill and others say, the thing works incredibly well and is mind-bogglingly reliable! Which it will be again very soon I hope . . .Groily

[Biscuit]

I have used gaskets twice with no problems, With respect to the cylinder base, paper gaskets may have been fine when the joint faces were new and pristine, I don't use them at all now. Get some 1/32" Hallite or similar from a model engineering supplier, I replace all the paper gaskets with this, including between the crankcase halves.

[Tolly]

Just a thought, have you measured the depth of the recess where the spigot fits? If there is dirt/carbon or burned on oil it may stop before the head clamps the gasket.

Regards Tolly.

[Groily]

Thanks v much for the Hallite tip Biscuit - I'll get a supply in. Never tried it or similar before but sounds just the job.
Re carbon build-up, I was hoping I might have left some and that it would be the problem .. . but no, 'fraid not. Amazingly, I could read the +40 marking on the piston crowns after just a wipe with a rag, they were that clean. I had the spigot height on the cylinders at .115 or near enough using just the stick on a slide gauge, which ain't the greatest way of doing it, but accords with Biscuit's figs above. I'm sure there's nothing amiss there and of course it was working fine until I thought I ought to have a precautionary look at the valves (and indeed replace the exhausts). Guides, incidentally, absolutely superb - no wobble with new stems and precious little with the old ones - and I know they had done a good 60,000 miles. Outer valve springs all within 1/32nd of maker's free length and that was the worst one, inners closer still. Remarkable engines I think, and very rewarding to play with when it is very occasionally necessary. I'll be happier when I've got it nicely settled down again though!

[wilko]

What's the depth of the spigot recess in each head? That should tell you how thick the crush on the gasket is getting? Douglas