Boxerworks.com

Major Softie wrote:Almost no modern bikes are purely air-cooled except for some low performance small bikes. Pretty much all the current "air-cooled" engines have a oil-cooler and some sort of oil cooling of the exhaust seat area. I know the Ducs and BMW's are, and I think even Harley has gone that way, but they also have a much lower HP per cc design.

Might the fins be there to keep expansion rates similar between the head and nut. In a perfect world you don't want one heating or cooling faster than the other. This might prevent exhaust leakage when very cold or hot. On the other hand VW's, Porsches, etc. don't have them, Some aircraft do and some don't. All my flat 4 VW's have had more than there fair share of leaks at the head/exhaust union. Never had an exhaust leak there on my R90.

Interesting to note that BMW themselves reduced the size (and subsequent cooling area) of the exhaust nuts for the /7 series. Thus BMW themselves deemed these devices of less importance as essential cooling components to the 70bhp RS engine than the preceding 67bhp R90S engine.

Motorworks offer for sale a replacement clamp device for bikes with wrecked threads no longer capable of taking a nut, and that clearly offers no extra fins by way of additional cooling area. I suspect the reasons for the fins are convention (most air-cooled bikes have something similar) and to give the exhaust nut spanner something to grip.

DanielMc wrote: I suspect the reasons for the fins are convention (most air-cooled bikes have something similar) and to give the exhaust nut spanner something to grip.

"Exhaust nut spanner" That's a fancy phrase for a hammer and screwdriver Daniel!

DanielMc wrote:I suspect the reasons for the fins are convention (most air-cooled bikes have something similar) and to give the exhaust nut spanner something to grip.

This is the weakest explanation/speculation I've heard on this thread - by a long shot. There's an awful lot of better ways to give a wrench a way to grip that nut than putting fins on it, and they've all been done.

The fins were not placed on the exhaust nut in order to give a wrench a way to get the nut off. Pure aesthetics is way more likely than this idea.

ME 109 wrote:

DanielMc wrote: I suspect the reasons for the fins are convention (most air-cooled bikes have something similar) and to give the exhaust nut spanner something to grip.

"Exhaust nut spanner" That's a fancy phrase for a hammer and screwdriver Daniel!

Now be careful Jeff, Duane is still recovering.

justoneoftheguys wrote: Now be careful Jeff, Duane is still recovering.

I can show Duane a picture of my nuts if his pulse goes up.
Should be enough to bring it back to normal.
Diane, dunno.

LOL, no sympathy on this forum.

DanielMc wrote:Interesting to note that BMW themselves reduced the size (and subsequent cooling area) of the exhaust nuts for the /7 series. Thus BMW themselves deemed these devices of less importance as essential cooling components to the 70bhp RS engine than the preceding 67bhp R90S engine.

Motorworks offer for sale a replacement clamp device for bikes with wrecked threads no longer capable of taking a nut, and that clearly offers no extra fins by way of additional cooling area. I suspect the reasons for the fins are convention (most air-cooled bikes have something similar) and to give the exhaust nut spanner something to grip.

Yes, pre-77 airhead riders have bigger nuts. I still like my heat expansion theory. The larger finned nuts and the earlier valve covers were probably more expensive to cast as well. Small changes could add up to bigger profits.

Yes, pre-77 airhead riders have bigger nuts. I still like my heat expansion theory. The larger finned nuts and the earlier valve covers were probably more expensive to cast as well. Small changes could add up to bigger profits.[/quote]

I guess you gotta have larger nuts with that earls front end in cornering situations!