Help with finding engine number and identifying - R100

Discussion in 'Airheads' started by Patrick M, Nov 11, 2012.

  1. Patrick M

    Patrick M Been here awhile

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    I bought an R80 G/S yesterday and was told after the fact that it has a complete R100 engine in it. I was a little annoyed at first that I didnt know this before hand but am over it now. The bike rips.

    I definitely see myself doing a thorough rebuild on some level in the future and I need to know exactly what the engine is. I looked near the oil filler but did not see anything stamped there. Is there another place that I may be able to find an engine number and hopefully be able to pinpoint its production year/type?
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  2. Airhead Wrangler

    Airhead Wrangler Long timer

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    lower front left side there's a milled flat on the case that has the engine number on it.
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  3. gsd4me

    gsd4me 90% bluff

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    It should do, all that grunt with the R80 lower gearing. If you need to pull the gearbox in the future it may be worth fitting a 5% higher top gear, and if you plan to use it for gnarly work, a 5% lower first gear may help as well.
    #3
  4. Patrick M

    Patrick M Been here awhile

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    Yes, when I do the transmission I'll be replacing 5th with taller gear.

    Thanks for the help, I'm new to airheads... I found the number but I don't think it tells me much about the engine type/year. 05/95/0087 is what is stamped on the milled flat.

    This particular R100 engine has oil lines running to the heads but there is no oil cooler. Was there ever a production that used this design? Or could the oil cooler have been left off during a conversion?
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  5. Airhead Wrangler

    Airhead Wrangler Long timer

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    The blocks are not labeled for displacement because the same bottom end was used on both R100 and R80 models. That production date stamped on the engine indicates that it's a VERY late production block made in the 5th week of 1995 and was the 87th block produced that week. That's consistent with the lack of serial number stamped at the oil filler as they stopped doing that in late '83 or early '84 or thereabouts. It looks like your bike got a complete motor swap at some point during its life. I'm not sure what you mean by the oil lines running to the heads. That's not normal. Care to share some photos? Also, what markings are stamped at the cylinder base? That can tell us if you have BMW cylinders or aftermarket siebenrock cylinders. Since the block isn't original though, it sounds like someone just swapped in a complete R100 engine. The lack of oil cooler isn't a huge problem. Some people see them as more of a liability on GS or G/S models and leave them off. There were plenty of much hotter running, higher compression, faired models without oil coolers that did just fine.
    #5
  6. R100RT Mark

    R100RT Mark Been here awhile

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    Probably not oil lines. Probably the pulse-air system secondary air system. Images will confirm.
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  7. Patrick M

    Patrick M Been here awhile

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    [​IMG]

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    There was a complete engine swap at some point. See OP. Thanks again for the help.
    #7
  8. Airhead Wrangler

    Airhead Wrangler Long timer

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    Mark called it. Those aren't oil lines, that's the "pulse air system." It feeds air into your exhaust port on over run to help any unburnt mixture burn as it flows out your headers. A lot of people rip this system out as it doesn't do much except clutter up the bike and any environmental benefit is probably negligable. Removal involves getting rid of the lines as well as some vacuum valves inside your airbox. If you want to remove the system, order some oil drain plugs (I think from 1150 model?) with crush washers to plug up the holes on your heads and then two airhead timing hole plugs to plug up the holes in your airbox. The vacuum ports on your carbs can be plugged with either screws (can't remember the thread side, but that barb fitting IS threaded on the inside) or standard automotive rubber vacuum caps.
    #8
  9. Patrick M

    Patrick M Been here awhile

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    Nice! Thanks for the tip. I'll consider that once I get into the bike some more...

    So it's looking like this engine could either be a mid-90s R100GS, R100R, or R100RT. I'm basing this assumption off of the stamping and what 1000cc airheads were produced during that time.
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  10. Patrick M

    Patrick M Been here awhile

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    I suppose this also means that my speedometer is inaccurate, unless the R80 gearbox was used during the swap. Looks like I have some diggin' to do.
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  11. Airhead Wrangler

    Airhead Wrangler Long timer

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    R80s and R100s (and R65s and R45s, for that matter) used the same transmission. It's your final drive ratio that determines speedo ratio. Seeing that you still have the stock 3 lug monolever final drive I can almost guarantee that you have a 37:11 final drive. There was only one other ratio available in a 3 lug final drive, 32:10, which was available as a factory option for the R80ST (but would also fit the G/S), but they are very rare to find. They're still available new from the factory though. I bet you'll find 37:11 stamped right next to the swingarm oil fill port on your final drive.
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  12. Patrick M

    Patrick M Been here awhile

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    Ah, gotcha. Thanks for all the info, man. And yes, I just checked, it's a 37:11 ratio...

    So if Im understanding this clearly, the essential differences between an R100 engine and a stock R80 is the top end parts and oil capacity? Gearbox and bottom-end are the same for each.
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  13. Airhead Wrangler

    Airhead Wrangler Long timer

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    Bottom end, clutch, and gearbox are identical, same part numbers. The only reason for the difference in oil capacity is the oil cooler that required just a bit more to fill.
    #13
  14. Rucksta

    Rucksta SS Blowhard

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    G/S has a shallow sump with 4 mounting points for a skid plate
    Dipstick is different on a G/S.

    Hard to see in the photo, your sump may be the shallow one already.
    Oil capacity is 2lts or 2,25 ltrs with a filter change to the top mark
    #14
  15. Airhead Wrangler

    Airhead Wrangler Long timer

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    See above, the motor is from '95, so it could be a later GS sump.
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  16. boxerboy81

    boxerboy81 Stay Horizontal

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    Is there a number stamped adjacent to the oil fill hole next to the left carb?
    #16