TECH: Fix Broken Fins

[Guest]

I always hear people use the term "chipped" fins when they are selling a set of heads with busted off hunks of metal. Lets call it what it is...broken fins. I was putting one of my motors back together when I realized it has a couple broken fins. Somehow I don't remember these being like this when the bike was being ridden, but either way, they're broke now. Since I have to fix them, I figured I would post some pics of the process of fixing them. I'll keep adding a post to show the progress as I go.

Here's the broken fins. The top one is completely gone and the lower one is missing a chunk.

[Guest]

Here's the start of the repair. Make a paper template of the broken fin and transfer it to the aluminum. I used a chunk of .125" aluminum scrap. The fins are technically closer to .150", but honestly, no one will know if you don't tell them.

Another option to this, although a bit more complicated, would be to cast a piece. Since I have the ability to do casting, it was tempting. Honestly though, I'm lazy and it's not a perfect restoration, so I just don't feel like spending the extra time to do it. It would make a it a perfect fix though.

[Guest]

Now it's time to weld in the piece. I used a piece of .250" steel plate to space the fin from the top of the fin below it during welding. You need to chamfer the weld edge really good to get a good weld with as little seam at the bottom as possible. The beauty of welding HD castings is how wonderfully pourous and contaminated they are. It can take a few passes to get a good clean weld. I got pretty lucky on this one and got it with one pass using 4043 rod. I had already blasted these heads and put them in the oven for awhile to bake out as much of the oils and contaminants as possible. You can see the black crud that came out during welding. It wasn't bad enough to warrant grinding and re-welding, but sometimes it gets pretty annoying. Import bikes seem to have much better castings, and thus, less contamination. They are usually die cast as opposed to sand cast as well, which makes a big difference.

[Guest]

Now I carefully grind and shape the fin to match the rest of the head. It's looking pretty good, but it's still obvious that it has been repaired. After it's all shaped and finished, I throw it in the blaster to give it the cast look. Once it's done, it's virtually unnoticeable. I could have spent a few more minutes making it perfect, but again, this is going on my personal ride and the only one that will notice anything that's not perfect is me. All in all, pretty nice I think. Once it's on the motor and assembled, no one will be the wiser.

So the lesson here is, it's not rocket science to repair fins. Just a little time, patience and a welder. Hopefully this will help someone that wanted to do it themselves but just weren't sure how to do it.

If you don't have a welder and need this kind of work done, hit me up.

[Guest]

Yeah man. All it takes it a little talent, a few tools and the determination to do something for yourself instead of buying your way through life. DIY is part of the motorcycling experience. Good job, nice write up!

[jimthefred]

Very nice write up!

[GummoYuckmouth]

Consider this my nomination for the greatest hits category.

[specialseventynine]

Excellent job and write up.

[zero937]

very nice job man looks good

[gyro]

Damn Eric, you make it look too easy. I wish the Army would give me more time off so I could come hang out and follow you around your shop and pester you with questions all day.

[TimB1640]

great info !

[Jimbo]

rod

that 4043 is a nickle base rod right?? I need to do a repair on a ironhead with a crack in the rear motor mount boss on the rear head Thanks

[Bullneck]

That's an impressive fix toxic! The bead is tiny man! What kind of welder are you using for that! is it an inverter?

I have braised broken fins back onto an Ironhead before....but nothing like that. I'm gonna have to give it a try this weekend, thanks for the tip!

[Guest]

I use an ancient Airco tig welder. 4043 is an aluminum rod, primarily for castings and softer aluminum. 5000 series rods tend to crack when used on castings, but work well with billet.

Ironhead stuff would require a nickel rod, and a lot of preheating. How about someone more in the know on cast iron chimes in here? If I do a ironhead, I'll post up the process since it seems like a lot of guys here ride old sporties.

[scout41]

I use an ancient Airco tig welder. 4043 is an aluminum rod, primarily for castings and softer aluminum. 5000 series rods tend to crack when used on castings, but work well with billet.

Ironhead stuff would require a nickel rod, and a lot of preheating. How about someone more in the know on cast iron chimes in here? If I do a ironhead, I'll post up the process since it seems like a lot of guys here ride old sporties.

Thanks for the info. The only trouble I have with this is that your fins broke off in an easy (er) spot, and you have tig. What alternatives are there to tig? I'm going to pick up a flux mig later. The fins that are broke off are in weird spots on both my bikes, mid cylinder and far back, the spacing is close too.

I surfed around on the internet and saw a guy that pulled a shot kawasaki head, cut off a couple fins to fit, and JB welded them to the cylinder. Would that work? Would flux mig work?

JB weld seems like a hick fix, maybe I'd be better off having the whole thing taken over to be tigged? I have to have some other tig work done (just sissy and bars) but don't want to haul the motor/frame down there. They bother me, two big breaks right out in front mid cylinder