repair bent fins

How to repair a bent fin

by Duane Ausherman

This article applies to almost all models and years of BMW.

Think of all the times you have seen a bent fin on a cylinder head or cylinder. The vast majority can be bent back into position without much effort. If a fin breaks off, keep it, and when the head has to go in for machine work, include the piece of fin for welding. Some of these guys are so good that once welded on, ground, and blasted, you can’t tell where the seam is. Do not be concerned about the loss of cooling.

We once had a customer with an R75/5 with one head with four fins broken off on the top side. It was hideous, and he had been riding that way for years. The compression was still good, and there was no evidence of overheating. I do not recommend riding with that much cooling surface gone.

This head had only slightly bent fins, but it was the only one available for an example.  A fin can be bent far more than these and still be repaired.  Best if the head is mounted on the motorcycle, as that “holds it up” instead of using a vise as shown here.

These arrows show two bent fins on a BMW motorcycle head. They can be bent back into position.

Here you can see two bent fins.  They bent away from each other, so something got jammed between them.

A slender tire iron is used as a lever to correct the bent fin on a BMW motorcycle head.

The trick is to bend the fin very slowly.  This fin is only slightly bent, and I spent about 4-5 minutes slowly increasing pressure on the tire iron.  Could I bend it back quickly?  I don’t have the nerve to find out.  I prefer doing this when the ambient temperature is fairly high.  I would not have a problem with first heating it up to around boiling.

This shows my hand pressure on the tire iron on the larger bend of the BMW motorcycle head.

I have corrected the first slightly bent fin and am now on the second one.  This one takes more pressure.  At first, I used two tire irons about 1 inch apart for leverage.  As the central part of the bend was corrected, I used only one tire iron on the worst part of the bend.

The BMW motorcycle head fin is now straight.

The fin appears to be still bent, but it isn’t.  The distortion that you see is a flattening of the top of the fin.  Now it is time to file off the mushroomed top of the fin on the left side.  That will show nice shiny aluminum where it was filed.  The aluminum will corrode to match the other fins in a few months.

Start on your head with the least bent fin for practice. Use something for a lever that is very strong and thin.  I use a Dowidat tire iron.  Apply pressure very slowly and evenly.  If you are concerned about breaking off a fin, then try using a wedge and leave it overnight.  You are using what is called “cold flow” to allow the correction slowly.  This procedure is similar to straightening a clutch or brake lever.

Updated 11 July 2022