Lighter oilhead wristpins in airhead engine (R100GS)

Discussion in 'GS Boxers' started by drjason, Apr 8, 2014.

  1. drjason

    drjason awesome

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    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.

    [​IMG][​IMG]


    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)?
    #1
  2. def

    def Ginger th wonder dog

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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.
    #2
  3. Marki_GSA

    Marki_GSA Long timer

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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.
    #3
  4. Marki_GSA

    Marki_GSA Long timer

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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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  5. MassiveLee

    MassiveLee Banned

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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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  6. Marki_GSA

    Marki_GSA Long timer

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    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/produ...en&info=p278_Boxer-crankshaft-balancing.html& just the first link that came up.
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  7. MassiveLee

    MassiveLee Banned

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    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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  8. BMurr

    BMurr Been here awhile

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    That makes a lot of sense, but then what happens when you install those siebenrock upgrade kits with lighter pistons. Could it be that the boxer design balances itself to some extent such that a drop in weight such as that with the siebenrock kits will go unnoticed? Confused
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  9. def

    def Ginger th wonder dog

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    I'm still struggling with the chrome being removed...what chrome?
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  10. Chat Lunatique

    Chat Lunatique aka El Gato Loco

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    Naw...Lee has it right. The counterbalance is for the crank imbalance only. As long as the left and right rod/piston ass'y mass are identical, they have no effect on balance.

    Read your link again. Counterbalance is for a rotating mass like the crank, pistons are an oscillating mass, 180 degree opposed = cancelling.
    #10
  11. Dan Cãta

    Dan Cãta Long timer

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    So.. did anyone try this yet? Is it worth the trouble?

    Dan.
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  12. GrouchyGeezer

    GrouchyGeezer Not a Long Timer

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    I'm with def on this one!

    Also, "carb cleaner removes chrome", huh ...what? What in heaven's name are you using for carb cleaner? :eek1
    #12
  13. lkchris

    lkchris Albuquerque

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    Lightweight piston pins for Airheads has been around for decades.

    Never heard of anyone using Oilhead pins, however.

    This is mostly applicable to the 1000 Airheads, as the smaller displacement motors exhibit hardly any vibration. I never minded the feel of the stock 1000 anyway.
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  14. def

    def Ginger th wonder dog

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    Balancing a piston engine is a science that few understand well. There are several factors in play as the engine rotates as well as rocking moments. Installing a rotating or reciprocating item that is not sympathetic with the engine balance will ruin your day.
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  15. def

    def Ginger th wonder dog

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  16. thirsty 1

    thirsty 1 Rider

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    If your reducing the weight evenly on both piston assm. you will not have a problem with balance. I also don't see a problem with the strength of the pins considering that the oil heads output power is greater then the air head. Id' say go for it.
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  17. def

    def Ginger th wonder dog

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    Not true. Adding or subtracting to the reciprocating weight will effect both first and second order balance and increase the rocking moment.
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  18. thirsty 1

    thirsty 1 Rider

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    Ya know your right. The PO will also need to remove the same amount of weight from the rod big end to keep everything hunky dory.

    A rocking plane vibration would be in the 4th, 6th, 14th, or 16th order. A1st order vib would be 4times the engine speed
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  19. Chat Lunatique

    Chat Lunatique aka El Gato Loco

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    Good article on balance def. I want to quote a line from page 17 on opposed twins.

    " because both pistons are always working in opposite directions, both primary and secondary forces are totally cancelled leaving only primary and secondary couples due to the offset between the two cylinders, figure 14."

    This couple due to offset is very small. Boxers ran for years with no counterbalance. If you look a the counterbalance shaft mass in the clutch area of modern boxers, the moment of the CB weight ( in gram/centimeters) that counteracts this couple is very small. The effect of the change in mass in the wrist pins will have only a very small effect here, if any.

    IMHO the counterbalance shaft is there to nullify the effect of the piston offset on the crank, not the oscillation mass. What do you think?
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  20. def

    def Ginger th wonder dog

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    I have never been inside of a modern boxer engine (oilhead) and so, have not had the opportunity to study the rotating assembly up close and personal.

    But, I do know this, messing with component weight inside any piston engine carries with it the risk of creating problems. BMW has made changes to the rotating assembly in the modern boxer engine as power, RPM and engine size has increased. I'm going to trust that they have also made the needed balance changes to accommodate these internal engine changes along with determining the appropriate RPM limits imposed in the ECU.

    I would not swap wrist pins unless you also rebalance the engine.

    Next question, what balance factor would you choose for the new balance? I do not know what BMW chose for the balance factor and so, I'll leave things alone.

    Vibrations cause all kinds of issues with bearings, chassis and the whole motorcycle. Metal fatigue may be introduced with dangerous results.

    I recall two connecting rod failures in late 50s Triumph 650 twins that occurred because the owner had fitted new forged pistons (equal weights but not the same as the OE pistons).

    This introduced some stresses near the big end of the RH connecting rod and caused the rod to part (the OE Triumph conrods were massive alloy assemblies and very tough). Both these engine were rebalanced and they never had the problems again.

    IMO, this wrist pin thing is an opportunity for disaster.
    #20