How to repair a bent fin
This article applies to almost all models and years of BMW.
Think of all of the times that you have seen a bent fin on a 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 that had one head with 4 fins broken off on the top side. It was really ugly 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.
Here you can see two bent fins. They have bent away from each other, so something got jammed in between them.
The trick is to bend the fin very slowly. This fin is only a little bit bent and I spent about 4-5 minutes with slowly increasing pressure on the tire iron. Could I just 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 heating it up to around boiling.
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 main part of the bend was corrected, I use only one tire iron on the worst part of the bend.
The fin appears to still be 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. If that bothers one, then just have it glass blasted and the whole head will look great again.
Start on your head with the least bent fin for practice. Use something for a lever that is very strong and thin. Shown is 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 slowly allow the correction. This is very similar to straightening a clutch or brake lever.