airhead parts - the dark secrets revealed

Discussion in 'Airheads' started by datchew, Feb 24, 2007.

  1. Plaka

    Plaka Brevis illi vita est

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    I've looked at some. Unfortunately it's not stainless. Appreciate the tip. Might find some model watercraft stuff that will work.
  2. Les_Garten

    Les_Garten Been here awhile

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    Perhaps Mr. Gasket or somebody like that.
  3. baldwithglasses

    baldwithglasses Godspeed, Robert

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    Hell naw, it's Kennesaw!
    Again, the R65 is NOT an R100. It LOVES REVS. Surely someone has a dyno printout of an R65 vs an R100 in similar tune.

    But no matter how hard you try, an R65 WILL NOT drive like an R100 or even an R80. Please believe people when they tell you this.
  4. motu

    motu Loose Pre Unit

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    I think that's the problem with peoples impression of the R65 - they come to it expecting it to be like other BMW twins....and it doesn't match expectations. For me I come to it from 650 2 valve pushrod twin experience - and for a engine of that type in the high 40's to low 50's HP range it stacks up pretty well...a little overweight, but smooth with stable handling.
  5. Plaka

    Plaka Brevis illi vita est

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    Bolts don't fall out but the gaskets to feel too soft (I'm using two). Didn't leak but I'm replacing them with paper. Just didn't like the feel of the silicone. No, never fell out but it was just worrisome. No locktite but did use a skim of ultracopper with the gaskets and that extruded into the bolt holes and hung them up.
  6. Plaka

    Plaka Brevis illi vita est

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    I found some 3/32 SS rod at the local hardware store along with some nifty black plastic ball knobs. if I make closer fitting barrels it looks like it will work out.
  7. Les_Garten

    Les_Garten Been here awhile

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    Outstanding!
  8. erappaport

    erappaport pray for rain

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  9. Stan_R80/7

    Stan_R80/7 Beastly Gnarly

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  10. Pokie

    Pokie Just plain Pokie.

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    Braided fuel line in 7mm (listed specifically as fuel line), $2.00 a foot. Bug-eyed Air Cooled VW Parts, www.bugeyed.net FedEx shipping.
  11. apt13

    apt13 Been here awhile

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    is there an "autoparts store" replacement for a headlight relay on a 78 R80/7? the originals aren't terribly expensive, but was hoping i didn't have to order one and wait around for it.
  12. Plaka

    Plaka Brevis illi vita est

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    As best I can tell the "headlight relay" is actually a load shed relay for the Always-on headdlight bikes. Kills the headlight when you hit the starter. I put a switch on my headlight so just ditched the "headlight relay" and redid the wiring. No ill effects. I do have actual power relays on the headlight so it isn't drawing through the switch.

    I would only use the factory relay unless you cut your old one open and confirm a circuit you can get out of a stock auto store (power) light relay.
  13. supershaft

    supershaft because I can

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    Shorter rods make better mechanical efficiency, not less. The main advantage of longer rods is the different piston movement per crank rotation. Different length rods drastically changes the relationship of piston movement compared to crank movement. Shorter rods work well for low rpm and longer rods work well for higher rpm. Airheads already have a comparably long rod at 1.91:1. Longer rods (around 2.15:1 rod length ratio) via higher piston pin bores, not longer cylinders, increases torque and hp above around 4000 rpm because the pistons have more TDC dwell time.

    HD XR750's have around 2.15:1 rods. One of their secrets in making so much high rpm torque. After making them run smaller and smaller intake restrictors for four seasons I believe (three of the seasons in a row), modern OHC 1000cc engines are beginning to keep up.
  14. Beemerboff

    Beemerboff Long timer

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    There was a good article in an old Petersen mag on rod lengths.

    Working with a lightly modified 302 and changing nothing but the pistons/pin location they found that long rods worked better everywhere.

    Their reasoning was that with long rods the piston spent a shorter time at TDC , then moved away from TDC slower.
    So it was still to catch more of the pressure build up, and at the point where it could make most use of the pressure, when the angle between rod and the crankpin radius was around 90 degrees, the piston was further up the bore and subject to more pressure.

    All made sense to me, and of course the cams and possibly ignition timing could also be optimised to suit the longer rod too, putting it further ahead.

    But that is their opinion, and their results, and if anyone has a logical explanation as to why shorter rods work better, and some documented test results I am certain we would all like to hear them.
  15. DoktorT

    DoktorT BigBrowedNeandereer

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    Indeed, any controled variables testing by competent persons who publish results and conclusions for peer review are going to trump all the butt dynos in the world combined when it comes to REALITY. Just keep the salt grains available until you find such.
  16. supershaft

    supershaft because I can

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    Shorter rods do have advantages. For the most part greater mechanical leverage and less tendency to ping. At higher rpms increased piston dwell trumps those advantages. Most say around 4000rpm in a lot of engines including ours.

    Reality? I have worked with long rodded airheads. Butt dynos trump dynos all the time in reality but I have worked with long rodded airheads on the dyno too. From a stock 1.91:1 to around 2.15:1 works! If I had a real hot rod airhead, I would run long rods in a 94mm bore.
  17. Beemerboff

    Beemerboff Long timer

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    A longer rod always has a straighter push at the crankshaft, and that will translate into better mechanical advantage, not less.

    And the piston on the shorter rod spends more time at TDC where it isn't moving, and if it isn't moving it isn't making any power.

    It is hard to see any point where a short rod has any mechanical advantage that would enable to overcome these to disadvantages and get in front.

    Just why things change at 4000 revs also needs a bit more explanation.

    FWIW the optimum dimensions for a 500cc air cooled two valve motor running to around 7500 revs were determined around 60 years ago, all square at 86 mm , with a rod stroke ratio around 2 to 1 - BMW just lost the plot trying to achieve a narrow motor.

    A 2.15 rod ratio is certainly a step in the right direction, and would appear to partly alleviate the damage caused by the ultra short stroke.

    I will leave it to someone whoactually has had a long rod motor an a
    dyno to confirm if it is optimum.
    But they would probably running at higher revs where the reduced stress of the longer rod due to the straighter push would be a big advantage too - BMW crankcased are reputed to be quite flexible when you get the revs/ power up!

    Richie Moore would have some of the answers, but like most sensible, professional tuners he will probably keep his hard earned information for those who use his services.
  18. supershaft

    supershaft because I can

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    Like I have said, I have worked with long rodded airheads in the dyno room and out. It sounds like you should read up on it. Some tuners prefer shorter rods for more mechanical leverage. You have figured that backwards. Shorter rods have more angularity and that translates into mechanical advantage.

    Right at TDC? That I have never payed attention to. Right around TDC? Longer rods have the piston staying around TDC much longer. Increased TDC piston dwell.

    What changes above 4000rpm? The rpm. Flame front travel time has its limitations. RPM doesn't. When the revs get high, it's nice to have a piston that waits for the flame front to push on it. Therein is the greater higher rpm torque. Some tuners even think that when the piston finally does get moving at a greater speed than a shorter rod in order to make up time for hanging around TDC, it really helps enhance the pressure differential on the intake stroke thusly getting an advantage on not only the power stroke but the intake stroke as well. For the most part, it slows things down AND speeds things up all at the right times. That has its advantages for cam timing as well.

    Your 60 year old optimum dimensions for a 500cc have long since been proved erroneous. That statement isn't even worth discussing IMO. Look into Honda's last four stroke engine that was designed to compete with a two stroke of equal displacement. Even getting not to close to two strokes had Honda using dimensions far from that nonsense.

    Sensible Professional tuners? More than a couple have written books that cover everything they learned on this subject.
  19. ÖÖ.

    ÖÖ. Ajaa kuin mummo

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    Anyone knows the dimensions of R100CS -82 fork seals and steering bearing? Or maybe even SKF part numbers?
  20. Plaka

    Plaka Brevis illi vita est

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    Try snowbum. I just got all the carb o-ring sizes and part numbers. Guy likes his data...for better or worse.