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MG Midget and Sprite Technical - Alloy cover sealing


Bit of liberty, but my 1955 Matchless motorcycle has developed an oil leak on the timing cover joint with the crankcase. I know as MG enthusiasts we have similar problems, so how to fix properly?

I removed it beforehand, gave both surfaces a good clean back to meatl, bit of emery on perceived high spots, bit of wellseal joint compound, but still leaks oil

Thoughts include

It has no gasket - not meant to - so possibly make one. Bit of a pain...

Use a thin bead of silicone sealant - tempting....

Use thicker oil! Mono SAE 50 ordered, currently using 20/50

Anything else?


Use a glass plate and fine grinding paste to flatten? We used porthole glass if you can get hold of one. Then use "gasket eliminator" - normally orange but works really well.
David Brenchley

The glass trick works well but could be difficult if half of the component is on the bike.

My process for similar joints is to ensure no high spots or burrs, (very) lightly clean the edges with scotchbrite or similar, fully degrease both sides of the join with panel wipe/thinners etc, brush on a very light coating of Loctite copper coloured RTV (Halfords do it), bolt it all up and then clean off any excess with thinners.

The idea of degreasing throughly is so that if there is any 'squeeze out' on the inside it should stay put. It works for me on my bikes for most gasketless joints.

You could try making a gasket but are you sure it's ok to use one? If it's a timing cover won't it introduce end float on the timing gears?

I would think 20/50 is a bit thin when hot on an old bike but then I have used it in my T100. I'm currently running it in and will probably use SAE 40 when its ready. 50 will be a bit thick but maybe for summer use on an engine with some miles on it will be ok.

Whenever I take my old bikes to the local bike meet people are horrified by even the smallest oil leak, I usually point out that it isn't a proper bike if it doesn't leak oil!!

John Payne

I never had an oil tight Brit bike until I started using Blue Hylomar. It even sealed up an Enfield!
Check the timing cover is flat and not slightly twisted. They very often are, but I don't know why. A good way to check is to oil the face and gently lay a sheet of glass on it. Any gaps will be instantly obvious.
Lightly countersink all the screw holes in the cover and the crankcase. Just enough to make certain there's no burr at the edge and (just as importantly) that torquing the screw can't raise an edge. Take any suspicion of a burr from the edges using a sharp scraper (my favourite is ground from a worn-out file). Clean it all as John describes and apply a thin even coat of Blue Hylomar to the faces. Walk away and have your lunch or something for at least an hour before you assemble it. The instructions say something like leave it to dry, but leaving it an hour will pay dividends.

By the time your Matchie was made metal quality was better than in the lean years of the 30s and 40s but the tools were mostly worn out and often screw location could be a bit misaligned. And by now it's likely that the threads will be worn too. I have often had to drill out and helicoil threads in Brit engines to get things to line up properly. Villiers two strokes and Norton parallels being the usual offenders, but Ariels and BSAs seemed to be made of sterner stuff. I've often wondered if decades of vibration took a toll on the fastenings too.

I have known people to counter bore the screw holes and install O ring seals. Ivan Rhodes, the Velocette guru, was one but that's a bit OTT I think. And I remember one bloke who spent ages trying to get a JAP 500 single oil tight despite it being designed with a total loss oil system that intentionally dumped the oil rather than circulating it. He had it in a road frame, but it was a speedway engine only meant to run a mile at a time. One man's Great Idea is another man's WTF?


One of my machining instructors back in the early 1980s in Wichita Ks mentioned he used to work in a motorbike shop, likely late 1960s early 1970s, and they sold the likes of Triumph and BSA IIRC and he said in his experience they were well machined but not well assembled so leaked as supplied but if taken apart and reassembled according to the manual they could be leak free. I have no experience with motorbikes, my only 2 wheeled transport has pedals.
David Billington

I support Grey, try Hylomar Blue, it will never harden, so it preserve the flexibility.
I have used "Form a Gasket" since mid 70'ties on old Britbikes with good result. It will stay flexible for some time, but then harden. I continued using it on the midget I bought in 2016.
Then I was introduced to the Hylomar Blue. At first I was very suspisious, but now it's my best friend and has outclassed the "Form a Gasket".
Try it, if it can seal an aeroplane jet engine, I'm quite sure it can seal your Matchless :-)
Jan Kruber


Many thanks. I have since given all the screw holes a deburr, filed and sanded down the high spots, and have ordered some gasket paper. Gasket should be fine - nothing locates into the cover. May supplement with Blue Hylomar - or possibly GP silicone....

If you consider applying sealing too, I would recommend the Hylomar in favor of the silicone. The silicone will harden, and allthough appearing soft after the hardening process, it's not sticky as the Hylomar continues to be.........
Jan Kruber

This thread was discussed between 19/04/2019 and 21/04/2019

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