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M20 head repairer?

Hi all,

I have a rather sorry looking iron head for my WM21. 11 fins are either missing or damaged. A welder today advised me to just buy a decent head. Sensible, but I don't like to junk original parts. Any recommendations for someone in the UK who may be able to fix it (preferably for a handful of magic beans)?


email (option): cmmacd @hotmail.co.uk

Re: M20 head repairer?

Taking a quick look a Ebay, there are four M20 heads for sale at the moment. Repairing broken fins can be very difficult if not impossible if you don't have enough room between the fins to weld the piece back on. You also need to make the missing fins out of another cast iron head or cylinder from something. By the time you are done you will be wishing you had just bought another head. Welders also don't work for free. I have repaired broken fins on bikes for which no parts are available, it is not fun.

Your logic is like trying to fix a bad big end bearing when you can buy a new one.

Re: M20 head repairer?

I have to agree and say it is not easy welding cast iron which also produces a toxic gas (cyanide I believe) I have successfully welded on fins in the past but only by breaking fins off another head and grinding to fit.
Eleven broken fins is just too much unless you are prepared to pay a lot of money. The head is just not worth the effort and expense IMHO.

Bear in mind the parts on the bike are unlikely all original anyway. My own 1941 WM20 was in Military use until about 1970 as far as I can tell and has a number of later parts fitted, probably in the MOD workshops.

Just purchase another one no-one will know and we won't tell.

email (option): tony-wilkinson@sky.com

Re: M20 head repairer?

OK, thanks both. Much appreciated.

All the iron heads I've found are damaged to some degree. I've gone for least worst and now have some donor fins.

email (option): cmmacd @hotmail.co.uk

Re: M20 head repairer?

I just welded mine with a stainless steel electrode. Simple welding that holds to this day.
I do remember having a bit of a hard time finding a piece of metal of the right thickness. Toxic gases? Wear a mask.

email (option): michasteinmann@gmail.com

Re: M20 head repairer?

Craig, I admire your ambition!

Lot's of people "restore" just by simply buying new parts (NOS or replica) without much craftsmanship involved, and in the end with not much of the original bike left. In this case however I agree with your welder and with what people have written i the thread. By another one. It is not that hard to weld some new fins on, but to get i to look good and to hold up for vibration and perhaps "the odd slip of the spark plug-wrench" it's quite a task.
And if you think of it the other head that you by could just as well have been the next one in the pile to be mounted. Had the workers just taken their toilet break in a different order the head you buy might have been the original part on your bike! :grinning:

Best regards,

Simon

Re: M20 head repairer?

Gas welding is still the best way to weld cast iron.

email (option): pes.sales@btconnect.com

Re: M20 head repairer?

There is a better than average chance that the head has been replaced in the first place.
Put the old head in your shed with a lable on it because there is a slight chance it will be repairable for a reasonable price one day.
Usually I would see 1/2 dozen M20 heads , short a few fins at every swap meet .

If you are building the bike to look at then leave the old head there for authentic patina
If you are intending to ride it to places that are more than about 1 hour away replace it.
Those iron heads get hot enough as it is let alone when missing 1/2 their fins.

Re: M20 head repairer?

I'm building the bike to ride, but want it to start out as good as I can make it. It'll have to earn its dents then.

I've seen a range of views about repairing old cast iron. The common factor looks to be the time and cost of repairing this head. I'll hang onto it: there may just be a series of Weld Off being planned.

As ever, thanks to all who've replied; it's appreciated!

email (option): cmmacd @hotmail.co.uk

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