T120 8-stud cylinder head flange depth - Triumph Forum: Triumph Rat Motorcycle Forums
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Classic, Vintage & Veteran For Coventry and Meriden Models. Anything pre-Hinckley goes.

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Old 11-28-2012, 03:03 PM   #1 (permalink)
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T120 8-stud cylinder head flange depth

Hi folks,
Been using my 8-stud (~1960 ish) T120 cylinder head for 32 years.
I've had it skimmed twice in that time, and it was probably skimmed once or more in the 20 years before I got it.
The result of this is that there is almost no flange remaining, in places the gasket face almost blends into the bottom of the lowest fin.
When running it over the last 5 years with 8.5 pistons 40 thou oversize, on the best (crap) modern petrol, though it went like a rocket, it was very "on edge" and fussy with plugs/carbs etc.
Compression gauge readings typically 185+psi.
Very rough calculations suggest that skimming 1mm would raise CR by approx 1/1, another 1mm by another 1/1 (not quite so much in fact, just roughly)
I wish to "soften" it somewhat, by using a thicker head gasket to replace some of the skimmed head flange.

As a starting point to judge how thick a gasket to use, I would like to know deep the head flange was to begin with.

Any of you who have an 8-stud (or even early 9-stud may help)head, could you give me an idea of the depth of the flange relative to the bottom fin, or from the rockerbox surface? Would be much appreciated.

Anyone with experience of thicker head gaskets, or even the merits of stacking gaskets together?

From the amount of squash I have on the pushrod tube rubbers, I reckon my head flange has lost 1-2 or more mm.

Many thanks Dave
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Old 11-28-2012, 05:42 PM   #2 (permalink)
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You probably realise that it's not good practice to ever mill a head on a Triumph twin.They will pull down flat if they're slightly bowed.They can be straightened,but it involves heating.Some say 0.007" bow is borderline,but I've seen them work with 0.010".
The main thing that bends them is too much pushrod tube seal crush;0.030"-0.040" was recommended.

I can only give you the overall thickness for a unit (9-bolt) head,but it should be the same.Barrel height,pushrod length,and rocker box geometry didn't change between pre-unit and unit.
A new head should measure 2.785" overall,at the bolt holes or rocker gasket surface.There are some variations and production tolerances.Some heads measure up at 2.797",some will be slightly less than 2.785".

A single thick gasket would be better than 2 thin ones,if possible.You might get out of trouble with an 0.080" gasket.
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Old 11-28-2012, 06:04 PM   #3 (permalink)
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Monty's Classics (Cornwall) have .080" copper head gaskets. You can find Monty's phone number by googling, he's got a website but don't bother emailing, phoning's better.
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Old 12-01-2012, 03:22 PM   #4 (permalink)
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Many thanks for the info folks
Bared the head this eve, so will measure it morrow, and get back.
As I described, the engine has been a bit of a fussy pain these last 5 years, scorching performance, 70 mpg, but easily fouled and harsh at lower revs.
The best I ever had it going was ~87-97, running thunderbird pistons (7.5/1?), revved like a TR65, well behaved with loads of hi rev power.
Even then I suspect head skimming was giving much higher than the stated compression, when I bought the engine (1980) it had been prepared for sidecar racing by Herstmonceaux M/cles, after 6 years I changed from their hicomp to the locomp pistons, with the result noted above.
I think the overly high comp issue is exascerbated by modern fuel maybe.
Cheers Dave
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Old 12-01-2012, 04:48 PM   #5 (permalink)
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High comp doesn't foul plugs, unless they're failing to fire. What sort of ignition does it use?

Is it safe to assume you're using E3134 cams? If low speed running is bad and plugs are fouling, try opening up the inlet valve clearances to 8 thou, just as an experiment.
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Old 12-08-2012, 02:18 PM   #6 (permalink)
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Eventually got round to it (been pre-occupied with main bearing issues).

My 8-stud head measures ~70.8mmm between rockerbox and head gasket surfaces.

Compared to Mr Pete's say 70.74+ and mirko's 71.2, doesn't represent anything like as much as I suspected.

I had thought much more had been lost, especially as when you see the small spacing between the bottom head fin and the top barrel fin, and how the head gasket surface blends into the bottom fin in places.

Don't know quite what to think, knowing that it has been skimmed several times.

Thanks to you folks for your input

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