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Old 04-08-2014, 01:11 PM   #1
drjason OP
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Lighter oilhead wristpins in airhead engine (R100GS)

NOTE: I just copied this from "The Garage" because I think it is more relevant in this forum. I googled this and searched ADVrider and IBMWR 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 oilhead wrist pin 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, 05:52 PM   #2
def
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Just a guess but, changing the reciprocating weight in your engine may upset the dynamic balance of the engine. Also, a weight change at the pin bosses may also upset the rocking moment somewhat.

I would consult with an engine balancing service before putting your wrist pins on a diet.
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Old 04-09-2014, 05:53 AM   #3
Marki_GSA
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I am with def on this one.
In a world where engines are balanced to a gram or less this is a huge reduction in weight. Great idea if you replaced both and got it all balanced. Bad idea if you just replace one and do nothing about the balance.
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Old 04-09-2014, 05:54 AM   #4
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Oh and you might find your pistons are much heavier than an oil heads so the lighter/thinner walled pin might not hold up to the task.
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Old 04-09-2014, 01:37 PM   #5
MassiveLee
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Weight the 1000GS pistons. If they are the same weight or lighter than an 1150GS piston, then go for it. On both sides. ;-)

In terms of balancing. The crankshaft is balanced on itself. It doesn't take in account the rods and pistons. If you fit a lighter pin, that is less weight for the connecting rods to keep moving back and forth. The lighter pin is most likley mandated to reduce weight because of the longer stroke on the 1150GS.
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Old 04-09-2014, 02:38 PM   #6
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Quote:
Originally Posted by MassiveLee View Post
Weight the 1000GS pistons. If they are the same weight or lighter than an 1150GS piston, then go for it. On both sides. ;-)

In terms of balancing. The crankshaft is balanced on itself. It doesn't take in account the rods and pistons. If you fit a lighter pin, that is less weight for the connecting rods to keep moving back and forth. The lighter pin is most likley mandated to reduce weight because of the longer stroke on the 1150GS.
That's is not correct. The lumps sticking out from the opposite side of the big end on a crankshaft is the counter balance. The crank very definitely is balanced to the rod and piston, even on a boxer.

http://www.motoren-israel.com/produc...alancing.html& just the first link that came up.
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Old 04-09-2014, 03:26 PM   #7
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Quote:
Originally Posted by Marki_GSA View Post
That's is not correct. The lumps sticking out from the opposite side of the big end on a crankshaft is the counter balance. The crank very definitely is balanced to the rod and piston, even on a boxer.

http://www.motoren-israel.com/produc...alancing.html& just the first link that came up.

For having built several BMW 4-pot race engines, I will disagree on the wording, but will agree that when the big ends of the connecting rods are added to the equation, it must remain balanced.

You want to balance the crankshaft by itself, and lighten it if you want. That's part one. Part two requires that all connecting rods weight the same as a unit (I used to do it to the tenth of gram), and that all big ends also weight the same as well as small ends.
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