I was the sploke who broke a set of M20 forks about month ago. I had a friend who has allot of metal experience fix them. He informs me that the forks are made of cast iron. We got another original set and checked them and sure enough they are made out of cast iron as well. My friend tells me thats why they were brazed. This is a suprize to me as others who have had a lot of experience with war bikes thought the forks were made of steel. Being made of cast explains why they broke when I fitted them with the wrong linkages and spring.

how do they cast thin wall hollow tube in cast iron?

email (option): deadsheds@yahoo.com

My thoughts as well but he took some pictures of the inside of the tubing and it looks like cast iron and when you score the metal with a file you can tell if they are steel or not. So my friend says and it certainly looks like cast to me particularly where the forks cracked it was not like a crack in steel it was more granulated.

Hi Bryce...Cast iron yokes, lugs and other parts will be malleable iron castings...
This material does not share at all the brittleness of grey cast iron castings, as the heat treatment process applied after casting changes the internal structure and hence properties of the material...Grey cast iron would be entirely unsuitable for this application....

Malleable iron castings have good tensile strength and are resistant to shock loadings, having the ability to flex without breaking. In fact they are more or less compatible with mild steel in this condition....Totally unlike grey cast iron in fact...

Brazing, particularly in a 'tube and socket' type of joint would be a normal method of joining the parts together with either cast iron, malleable iron or mild steel components or any combination of the three...

The tubular sections of the forks are steel, not cast iron.

Therefore, I'm afraid your friends assessment of the materials used is incorrect, firstly in not appreciating that grey cast irons and malleable cast irons have completely different properties and secondly in incorrectly identifying the material used for the tubular sections of the forks...

As such it doesn't represent an explanation of why your forks failed, the reasons for which lie elsewhere...Ian

email (option): ian@wright52.plus.com

Thanks Ian...what you have written makes senses to me as we'll. I guess the cast effect that we saw in the top end of tubing was the malluable iron casts that you speak of. Anyway he did a bloody good job of repairing the forks.