Tuesday, June 3, 2014

Gasket time...

When I took the "modified" head off my engine, I thought I would re-use the head gasket that had been fitted. After all, that's the advantage of a solid copper gasket, you just anneal it and you're good to go.
That said, the old copper gasket had evidently gotten a little mangled around the pushrod tunnels (by no means unheard of, and not really a problem) and there was something else to consider: the new head had visibly less meat on the mating surface (although the step in the combustion chamber measured the correct .120") and as you know I've had it skimmed to ensure a truly flat surface for a proper seal. So, what we have here is a thicker solid copper gasket made by Jim Schmidt, specifically the 0.62" item.

The extra thickness is to compensate for the thin mating flange and its subsequent skimming. Additionally, it should allow adequate clearance for the heat insulating washers I've fitted under the intake valves and possibly lower the compression ratio slightly, though to be sure of that I would have to test it once the engine is back together and fully torqued down. Incidentally, jabbing your pinkie in the spark plug hole is not an accurate compression test. Get a gauge and measure properly.
Word of warning: it should be noted that the topic of head gaskets is one of those technical topics (like "which engine oil?") that inflames the Norton world, so don't be surprised if you can't find a definitive answer.
The first thing to do is anneal the gasket: after it's heated to a dull red, I leave it to cool on the bench and get on with the rest. Apologies for the rather cryptic image but I was by myself in the dark, and nothing good ever happens when it's like that:

This gasket came with some very thin copper wire, I've never tried this before but the idea is that it will provide a seal against the typical oil seepage that most Commandos experience around the pushrod tunnels and oil return hole. I start by applying a thin layer of sealant on the underside of the cylinder head, then lay down the wire in a loop, again apologies for the crappy photo:

The gasket, now cool, also gets an even layer of sealant on both sides, and then goes onto the cylinder.
We're finally ready for the head to go back on now.

2 comments:

Deanna Jones said...

I've also made the mistake of jabbing my pinkie into the spark plug hole. I don't think I'll be doing that again. How long should I let the gasket cool? I heated the copper gasket I was working on last time and I was too impatient to let it cool down long enough, so it didn't turn out the way I wanted.
Deanna R. Jones | http://www.dieandgasket.com/copper-gaskets.html

Artie said...

Hey Deanna, I left mine to cool overnight on a very flat surface. It has to cool down gradually in standing air (no fans, AC or anything like that) until it's safe to touch. Good luck!

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