Broken aluminium cooling fins on aircooled engine

I bought a broken 5 hp air cooled Yamaha 2-stroke outboard engine. So spark and such. (it helps if you’ve got a clip for an deadmans switch) Anyways, got it to run nicely. However, the cilinder head has cooling fins on it. And 2 of them were broken/still on there. But when I tryed to bend them back in their stock position, I suddenly had 2 pieces of aluminium in my hands.

Considering I’m going to use this engine for an project, where it goes into a space with little airflow, I want to have all the cooling capacity. This engine is probably at least 35 years old. I can’t weld aluminium, and even if I could, I wouldn’t attempt it. Very brittle and such.

I can try to drill the pieces, and attempt to bolt them back on. I’m however not too sure on how effective this would be, and how well it would connect.

Second option is using epoxy intended for metal. Looking at “Pattex Power Epoxy Super Mix Metal”. A brand called “Bison” has a scimilar product. However, the Pattex version goes up to 150 degrees C, while Bison only goes to 100 degrees C.

How hot does an aircooled engine (preferably like mine, which is 2-stroke and revs up to about 5500 rpm) get? Would this epoxy stuffs be a good idea? Or should I just try to find the money to get a replacement cilinder head. (the engine runs just fine, vid can be found here: )

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Hello Jahee .

Could you take a small 12 volt fan and concentrate the air flow where the fins have broken of ?


There are already (plastic) fins ontop of the flywheel, to blow (fresh) air into a shroud, which covers the cooling fins. A 12 volt fan (like used in pc’s) has nothing on the stock airflow.


Random example image taken from the web. (dark outside, not going to take a picture myself)


I would pick up some of the aluminum welding rods and use torches to weld them back on it is basically brazing and not very hard to do. I would definitely try it before I would replace the head. As to the liquid metal products or those puddy epoxy products I have not had great luck with them myself. Definitely don’t get them around gas it will eat JB weld even the stuff rated for fuel tanks doesn’t hold up.
Check out this video about aluminum welding

I have used this product with good results it is really not a hard process.


Thanks! This was so darn interesting! And considering the engine is really dirty and that there is a solid risk of fuel/oil spills, I won’t go with the epoxy products then. Luckily for me, this (final) outboard project is the 3rd of my medium-sized projects, so I can save up some moneys for a gas burner and such. (didn’t know brazing aluminium was a thing)