Magneto to engine joint

Flatspot · Aug 2, 2025

Is there supposed to be a gasket between the magneto and engine on a pre-unit Triumph motor? TR6/T120. Or is it simply a face joint with a little bit of gasket goo?

The builder of my motor can’t get the advance unit to tighten onto the magneto. The picture sent to me seems to have been fitted with a gasket.

bmepkid · Aug 2, 2025

Tell your builder to run a 3/8-20 bottoming tap through the advance unit nut. Often, some of the threads in the nut get filled up up with crud or a thread gets a little distorted. I've had many people ask me about this after I've done an overhaul for them.

Flatspot · Aug 2, 2025

OK, I’ll mention it to him. But is there a gasket?

baz · Aug 2, 2025

Flatspot said:
OK, I’ll mention it to him. But is there a gasket?

I've always used a gasket on a Magneto flange

Matt Spencer · Aug 2, 2025

Somebody might lap the tapers .

incidently , levering the drive gear off in a no-no .
if youve got three hands , undoing , so the gearwheels on the case ,
holding mag , and whacking the Mag Shaft with a aluminum drift ,
might be acceptable .

About 56 they went to a bigger bearing . BORE . So the brass thingamy bob is greater strength , thickness , O.D. .

The eaRLIER ONES WOULD BE MORE SUSCEPTABLE TO CRACKING / BREAKING THE (oops ) Brass sleeve where the steel tapered shaft fitsin .
So , for racing , use the bigger bearing post 1956 magneto . Was the romour that the G45 / Porcipine , with light cranks , twisted them off .
Hence the later mod. and the use of SR II mags .

If youre sure its gunna stay put , throw on a smear off sealant . Theres a long fastener for the lower inacessable nut . which is longer than the mag .

from : https://www.classicbikepartscheshire.com/electrical-c3/magneto-spares-c5/triumph-t1

Triton Thrasher · Aug 3, 2025

The gasket sets I’ve bought have contained a paper gasket for the magneto flange mounting.

Sealant and no gasket usually works though.

Fast Eddie · Aug 3, 2025

Flatspot said:
OK, I’ll mention it to him. But is there a gasket?

Yes.

However, if both surfaces are flat then it will seal perfectly well with silicone (or similar) and no gasket.

baz · Aug 3, 2025

I always use a greased gasket and no sealant
I'd only use sealant on a gasket if the surfaces can not be made good enough

Flatspot · Aug 3, 2025

I assume when a gasket is to be used it will be a thin paper one, rather than something thick like a rocker box gasket? Presumably a thicker gasket won’t allow the a/r unit to draw down to the taper correctly. I think this is what has happened. It’s a damn shame I’m so far away and can’t go and just ‘look’ at it.

Fast Eddie · Aug 3, 2025

Flatspot said:
I assume when a gasket is to be used it will be a thin paper one, rather than something thick like a rocker box gasket? Presumably a thicker gasket won’t allow the a/r unit to draw down to the taper correctly. I think this is what has happened. It’s a damn shame I’m so far away and can’t go and just ‘look’ at it.

Yes, standard is a thin paper gasket.

acotrel · Aug 3, 2025

The magneto looks as though it is sitting skewed. He might need to slacken the bolts and make sure the spigot fits into the case properly.

acotrel · Aug 3, 2025

Matt Spencer said:
Somebody might lap the tapers .

incidently , levering the drive gear off in a no-no .
if youve got three hands , undoing , so the gearwheels on the case ,
holding mag , and whacking the Mag Shaft with a aluminum drift ,
might be acceptable .

About 56 they went to a bigger bearing . BORE . So the brass thingamy bob is greater strength , thickness , O.D. .

The eaRLIER ONES WOULD BE MORE SUSCEPTABLE TO CRACKING / BREAKING THE (oops ) Brass sleeve where the steel tapered shaft fitsin .
So , for racing , use the bigger bearing post 1956 magneto . Was the romour that the G45 / Porcipine , with light cranks , twisted them off .
Hence the later mod. and the use of SR II mags .

If youre sure its gunna stay put , throw on a smear off sealant . Theres a long fastener for the lower inacessable nut . which is longer than the mag .

from : https://www.classicbikepartscheshire.com/electrical-c3/magneto-spares-c5/triumph-t1

It is possible for the long fastener to brake the stud through vibration and fatigue.

baz · Aug 3, 2025

acotrel said:
It is possible for the long fastener to brake the stud through vibration and fatigue.

Yep they are always snapping from vibration and fatigue aren't they!!!

bmepkid · Aug 3, 2025

What I'm hearing is that the auto advance won't pull up onto the taper, not hitting the inside of the case. That's why I suggested running a tap in the auto advance nut. There is either some crud or distortion in the threads. I've never had one of those long bolts snap a mounting stud.

Flatspot · Aug 5, 2025

Well I have mentioned back to him what has been said on here. While I haven’t heard back what he’s done, it is now all fitted and ready for collection.

It still has the tatty mismatched rocker cover inspection caps. I let him have a set of 4 finned ones which are the wrong size. Too large a diameter, it’s on a 9 stud head, what caps do I need to look for?

Flatspot · Aug 5, 2025

I think because I have a 9 stud head, the rocker boxes will be unit (?) and there is a different cap 70-4610.
Oh well. Look out ebay there’s going to be a set of early John Tickle style ones for sale soon

Magneto to engine joint

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