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Old 04-08-2014, 10:19 AM   #1
drjason OP
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Lighter oilhead wrist pins in an airhead

I googled this and searched the forum and I couldn't find anyone having discussed this yet, but it seems like it is worth considering:

Success: Soaking piston in carb cleaner removes carbon very well.
Failure: Soaking wrist pin (gudgeon pin) in carb cleaner removes chrome very well.
Solution: Call re-psycle for a good used wrist pin. 11 25 1 335 474 GUDGEON PIN - 22mmOD X 13mmID X 74.31mmL

What arrived happened to be what I figure is an oilhead wrist pin. 11 25 1 341 365 GUDGEON PIN - 22mmOD x 17.5mmID x 74.31mmL.

It looks like it would fit just perfectly. The original wrist pin is 147grams, the oilhead wristpin is 83grams. Weight savings of 64g of reciprocating mass per piston.




Now I need to decide whether to send this back and order a proper airhead wrist pin, or keep it and ask for another for the other side.

Question#1: Is there a heat/stress reason that the oilhead wrist pin shouldn't be used in an airhead?

Question #2: Why hasn't this been discussed before (or is my Google-Fu just not up to snuff)?
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Old 04-08-2014, 03:07 PM   #2
Cogswell
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I think the airhead pistons are quite a bit heavier than the oilhead pistons, no ? There would be more stress on the thinner wrist pin.



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Old 04-08-2014, 09:25 PM   #3
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That weight difference would be worthwhile I think ! .

According to some info I dug up, the weight of a piston for a R100 airhead is
570 gr (http://www.zeebulon.de/Mot/bmw_allg.htm#Kolbenbolzen) , and the one for an 1150 GS is 585 gr....(http://www.advrider.com/forums/showthread.php?t=836329)
Mind you I did not measure myself.
But that would be not too big a difference in stress if you are not constantly bouncing of the rev-limiter.....
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Old 04-08-2014, 10:23 PM   #4
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seems like a good enough idea. :hmmm
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Old 04-09-2014, 05:43 AM   #5
bmwrench
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That looks to me to be an aftermarket lightweight pin for an Airhead. It's been a while since I had an Oilhead apart, but my memory is their pins are heavier than the one in the picture.

As far as I know BMW wristpins are not chromed. Are you sure yours is unusable?
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Old 04-09-2014, 06:06 AM   #6
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Carb cleaner may be aggressive, but I don't think that it can remove chrome.

There was a craze for fitting lighter wrist pins some years ago and many claims were made about the results, however some of the wrist pins had a habit of breaking.

Nowadays you can get the same result by fitting lightweight pistons, but without the breakages!

I would suggest that you review what you have got
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Old 04-09-2014, 06:51 AM   #7
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Lightened pins were sold thru the Luftmeister brand. I also have heard of them breaking. I had a new set and got rid of them.
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Old 04-09-2014, 08:47 AM   #8
bmwrench
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I never had a Lufmeister pin, which were made by Western Wristpin fail. I have heard-and only heard-that those sold by CC Products had a habit of breaking. These were made by a different manufacturer.
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Old 04-09-2014, 10:36 AM   #9
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But not actual BMW pins, right?
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Old 04-09-2014, 11:41 AM   #10
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I have seen aftermarket lightweight wristpins break but I didn't know what brand they were. The stock ones are way too beefy. A lot of even stock pins nowadays are tapered on the inside for lighter weight and strength were its needed.

Siebenrock claims their pistons are so much lighter but most all the weight savings is in the shorter wrist pins, not the pistons. Misleading AFAIAC.

All wrist pins are chromed and then centerless ground down to the a couple of .0001". Like fork tubes but a lot more accurately ground.

I still have stock pins in my bike. If I went for less weight, I would hone out some stock ones. Some of the aftermarket ones go too far in weight savings IMO.
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Old 04-09-2014, 11:49 AM   #11
bajaburro
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lighter wrist pin

i've had them in my 91gs for many years.best and cheapest thing you can do to improve a gs imho.
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Old 04-10-2014, 07:18 AM   #12
AntonLargiader
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Oilhead pistons are 59mm across the flats, while Airhead pistons are 67mm across the flats. That's not an Oilhead wrist pin.
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Old 04-10-2014, 09:49 AM   #13
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Quote:
Originally Posted by bajaburro View Post
i've had them in my 91gs for many years.best and cheapest thing you can do to improve a gs imho.
Care to expound on that? I'm reading these posts and thinking I'll bet I couldn't tell the difference if my gs had lightened wrist pins or not. But maybe I could.
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Old 04-10-2014, 01:17 PM   #14
BOETJE
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Quote:
Oilhead pistons are 59mm across the flats, while Airhead pistons are 67mm across the flats. That's not an Oilhead wrist pin.
But the OP measures 74.3 mm for his air head pin....

Quote:
i've had them in my 91gs for many years.best and cheapest thing you can do to improve a gs imho.
So you actually had oil head pins in your air head ?

I'm confused now .....
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Old 04-10-2014, 02:49 PM   #15
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The thin wall pins are clearly dimensionally compatible with my R100 Pistons. So they may not be oil head pins. I guessed based on the photos of the individual part on the fiche. Although the fiche lists size for airhead pins, it does not for the oil head.

I asked the shop to send me one more of each, and I'll decide on Sunday.

As for the plating, the chrome is definitely gone in the middle where it was exposed. I even took the buffing wheel to it to see if it would get shiny again. But it actually is textured and is unusable.
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