Hi all,

What is the best way to repair broken fins? I have the broken bits and was thinking I should braze them on.

Any tips are welcome.

Thanks!

email (option): britool51@hotmail.com

Hi Robb..It depends to a degree on where the broken fins are..'Top' or 'bottom' fins I usually have tig welded using a stainless filler rod. This produces a fairly good result after clean up.
If the fin damage is halfway down a barrel for example it is only possible to access the ends of the break with a Tig welder. Braze on the other hand will flow, and can be fed, along the entire length of the break.
Options in this case are to weld the ends and then run braze along the remainder or to braze the whole thing...Ian

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

I'd add to that for fins with small areas of damage on ends of fins etc "JB weld" (a high temporature epoxy) is extremely effective & never falls off with miles of use, its black, can be ground to shape & for small fin repairs can be held in shape while drying using cardboard either side of fin.
I swear by it , you can't see its been repaired & no heat input into the part.

Doesn't JB Weld (and similar) stop, or reduce, heat transference? Might look okay, but doesn't it make the barrel cool less effectively?

email (option): dannydefazio@sumpmagazine.com

Thanks Guys! I had thought of JB Weld, but was unsure. I think I'll have them welded and brazed as Ian suggests.

email (option): britool51@hotmail.com

Hi Danny, i don't think JB Weld is responsible for all the overheated M20's & burnt out exhaust valves i think its the actual design :-D
My post wasn't clear enough, i mean AFTER the main repairs you tidy up the small areas with it.

email (option): britool51@hotmail.com

When brazing a fin I put the head or barrel on a hotplate at about 350 Deg. F and let it heat up until very hot to the touch before brazing. After brazing I let the item cool very slowly by turning the hotplate down to lower temperature over several hours. It seems to help and have not had a braze crack yet. I am using oxy/atc in this case.

email (option): jrosson86@att.net

Hi Joe,

Where do you live? I'll send my stuff to you. Would you take specialty M20 tools in trade for work?

I just do not have these skills. Your method sounds really good to avoid temperature differencial cracking.

email (option): britool51@hotmail.com

Ian,

You said you tig the broken fin with stainless filler wire, how does this perform on the cast iron barrel? Or is that just as a tack for brazing? Also, would a brazed repair be suitable for cylinder head fins, or do the engines run too hot?

Cheers

Ben

email (option): mad_king_soup@hotmail.com

Hi Ben..No, the whole repair is done with the stainless rod. It works well on cast iron in my experience. Fully brazed repairs have no apparent effect on heat dissipation. I have had heads quite extensively repaired using this method....Ian

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

Cheers Ian

That's interesting, I have quite a few broken head fins where someone (or several someones!) have been overenthusiastic in the sparkplug area. I'll look into stainless tig.

Ben

email (option): mad_king_soup@hotmail.com

I have tig welded on a lot fins using a dissimilar rod, such as nickel or stainless, I've found that the best thing to do is pre heat the casting on a gas hob, or with a propane blowlamp, then as soon as you've finished welding it keep it heated with the blowlamp and slowly bring the heat down and after a while cover it with a bit of fibreglass wool until it is completely cool, sometimes they can crack even when they are touch warm, so insulating it is advisable. You can of course weld them cold, but it does increase the chances of it cracking as it cools. I have tried arc welding them with cast iron rods, but I never seem to have much success, so I'd stick with either mig or tig.

email (option): davmax@ntlworld.com

Good advice Dave...I had some problems with cracking during cooling before adopting the same method...Ian

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