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can anyone give me the thread sizes, diameter and thread pitch for the sump plug on a 1978 3 1/2 sport engine. on modern jap bike you can get magnetic sump plugs. so if i have thread sizes i can use one from a jap bike

Or glue a magnet to your plug....

thanks i thought of that, but which glue ? the glue needs to handle the heat and vibration while the engine is running and i need to know how hot the engine gets to pick a glue !

The original Harglo service manual says that the sump plug is 18mm

CC

I got some neodymium magnets from a firm called Cermag last year-nickel plated ones as it is corroded quite easily. I have araldited one under the sump plug, though it might have stayed put without. Didn't fancy it sticking onto the chain.
Anyhow I have so far not collected any manhole covers or scaffolding as I pass.They are about the diameter of the end of the sump plug and around 8mm thick. Put 2 together if you dare.You can get them apart with extreme effort and caution or you will get blood blisters as they snap together.Quite brittle too.
I should weigh the plug /residue to see if there is any increase but I have not.
I got 10 for about a tenner. If you want I can send one up. You will find the postman attached to the letterbox.

today i changed the engine oil and my 3-1/2 the sump plug size is 22mm dia X1.5 mm thread pitch and 18 mm spanner size !!

Let us know if you find something that fits.
I'm about to drill mine for lock wire and if I bung it up
I'll need a new one!

i found this one but its in US dollars which makes it expensive to ship to the uk.
we need M22 x 1.5mm thread pitch
order Number--- 1016 SM---

http://www.magneticdrainplugs.com/metric.htm

My sump plug looks more like the type shown as 1025 or 1009, though as it is in situ I cannot check the size.