bmw r80 t0 r100 conversion

Discussion in 'Airheads' started by enfielddnepr, May 4, 2012.

  1. enfielddnepr

    enfielddnepr Been here awhile

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    Dear folks, as an r80 is somehwqta cheaper then an r100, and the first will have less wear on the drive train as the r100 i have a quaestion about converting a 800 to 1000cc.
    I know about the siebenrock kits.
    quite expensive..The lower weight of the pistons is cool, will wear less on the bearings, but the higher compression i do not like.. want to keep octane 95 possible as fuel and don't like the extra wear..
    So, i think the most basic is swapping the pistons and barrel;s from a r100?
    but i guess the cylinder heads also need to be exchanged, the r100 head will have bigger valves? also carbs?
    finally the final drive will need a diffenrent set of crown- and pignon wheels..
    Does anyone here have experience with this?
    #1
  2. Airhead Wrangler

    Airhead Wrangler Long timer

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    First off, which year and model bike are you starting with? R100s post '85 or so use the same size valves as R80 models. Same 32mm bings as well. You can have R80 heads machined to fit R100 pistons, but naturally you'll need to find someone who knows what they're doing and it won't be cheap. Good used R100 heads can be had for $250-300ish. Barrels and pistons in the $400-500 neighborhood. By the time you're all in, you're well within the price range of a siebenrock kit, but for used components. All that said, pricing for you in Europe is probably completely different. The 9.5:1 compression ratio they use is just on the edge of doable for regular pump gas. Mid grade and premium should be fine. 95 should be absolutely no prob for you.
    #2
  3. enfielddnepr

    enfielddnepr Been here awhile

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    Hi, i was goning for the 1981-185 types.
    '95 is the lowest octane of fuel you can get here..
    #3
  4. Beater

    Beater The Bavarian Butcher

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    It depends on the years you're talking about.

    A short list would include:
    1. Pistons
    2. Cyclinders
    3. Heads: The heads are concave to the correct size of the cylinder ... so you need the right heads.

    Optional (depending on the years):
    4. Carbs: The 32mm Bings WILL WORK ... but the 40mm's will be better ... and Mikuni's (34's to 38's) are good too. If you go Mikuni call Rocky Point Cycle in MA, he can set you up with the right cables.
    5. Exhaust: If the heads are 'big valve' ... from the 77-79 range, the output spigot might be 40mm and not the 38 that was stock for most.

    Nice to have:
    6. Final drive in the 32 to 34 / 11 range. You probably have a 37/11 ... and if you're setting up a dirt bike ... then it will be fine. But for road ... you'll have a whole lot of pickup, and the engine will be screaming at 80mph. But a 37/11 will work.
    #4
  5. Airhead Wrangler

    Airhead Wrangler Long timer

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    Which octane rating system do you use in Europe? RON?

    EDIT: wikipedia to the rescue. It looks like you guys use RON. We use AKI. Our regular fuel which some people use in 9.5:1 engines without report of problems is 87 octane on our AKI rating which is equivalent to about 91-92 on the RON rating system. Therefore if your "regular" is 95 RON then it's better than our "regular."
    #5
  6. BeemerShop

    BeemerShop Ted Porter

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    If you don't like the 9.5:1 compression ratio of the Seibenrock kit, install one low test base gasket per side part number 11111335650 and that will drop compression to 8.8:1. The Seibenrock kit is the ticket.

    Seibenrock has a kit designed for the R80 with pistons machined to fit the heads so no head modifications necessary, makes it a bolt on kit. We sell and install a lot of them with great results.

    Ted Porter
    BeemerShop
    www.BeemerShop.com
    #6
  7. Boxer Metal

    Boxer Metal Mad Scientist

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    Last time I was looking for used post 81 pistons and cylinders it seems like they were all around $500.00. Then you have to clean them up buy rings($140.00) and more than likely replace the rusting pushrod tubes($120.00). So $720.00 vs $890.00 for new Siebenrock.
    #7
  8. BeemerShop

    BeemerShop Ted Porter

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    Hey there stranger. Yeah, and they're becoming harder to find. Congrats on Boxer Metal, cool shop.

    Ted Porter
    BeemerShop
    www.BeemerShop.com
    #8
  9. wirewrkr

    wirewrkr the thread-killer

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    All right you two, get a room! :lol3
    All we need now is for Bud P to pop in and we could have an old farts party!
    #9
  10. BeemerShop

    BeemerShop Ted Porter

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    He'll be along any minute the old fart.
    #10
  11. outfit

    outfit Been here awhile

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    So this was a real bargain
    http://www.ebay.co.uk/itm/BMW-R100R...98?pt=UK_Motorcycle_Parts&hash=item2a1c8cd2da

    Previous week I got a pair for £120 inc pistons. Thought I'd button it all up and see what the result was. I'm pleased to say the result was perfect. Just put on the usual new gasket's and rubber's.
    So I'd say keep an eye open on ebay and be as good a judge of what you see as is possible and then go for it.
    #11
  12. Beater

    Beater The Bavarian Butcher

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    That would have been a real bargan in the US ... I have no idea of the market in Europe ...
    #12
  13. lkchris

    lkchris Albuquerque

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    You're really better off just finding an R100.

    Because you really should match your "new" R100 engine with the same rear drive ratio that BMW typically fits. R100 won't be as smooth as an R80 and won't be so comfortable always spinning at R80 rpms.
    #13
  14. Airhead Wrangler

    Airhead Wrangler Long timer

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    Yeah, I got ahead of myself there. Since you don't even yet have this bike, I'd say you'd be better off just starting with an R100 to get a suitable final drive ratio. Just be patient. The right one will come along.
    #14
  15. bmwrench

    bmwrench Long timer

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    Will not!
    #15
  16. wirewrkr

    wirewrkr the thread-killer

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    AYUHH!
    Knew it, I did.
    #16
  17. tomjrtsmith

    tomjrtsmith n00b

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    Hey Ted,

    I'm thinking of this upgrade for my 1988 R80RT. I do a lot of highway miles and already find the fifth gear quite short. What would you suggest as the best final drive ratio to accompany the upgrade kit? I'd ideally like to sit at around 60mph somewhere around 3750 rpm but at present the motor runs around 4200 at this speed. There seems to be a huge amount of debate around how to go about this. Can you buy different drive ratios from BMW without having to change the entire final drive housing? This would seem to be the easiest option. It seems odd to me that there is every aftermarket accessory under the sun for airheads except for final drive ratios, which from the look of things is what many owners seem to end up wanting.
    #17
  18. Airhead Wrangler

    Airhead Wrangler Long timer

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    You can buy different gear sets, but once you include the labor of swapping it into your housing, you're better off just looking for a good used final drive from an R100 mono. Your other option is the 5% taller 5th gear available through various sources. Moorespeed also does a 10% higher 5th.
    #18
  19. Larry Landes

    Larry Landes Adventure Rider Dude

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    Seems I found the right thread, so I have a question...

    My brother and I both own '81 R80GSs. We used to ride them a lot but then he broke a valve. Since I'm the mechanic of the two and because he didn't have money at the time, I pulled the right head off of my GS and gave to him, thinking I was going to upgrade to 1000cc.

    It never happened and my bike has sat for far too long. But now he and I want to ride them from California to the East Coast and back, so it's time to get the old girl going again.

    Long ago I bought a really rough parts bike '83 R100GS. I've never touched it, but it's available for parts.

    So the question is, which option do I take:

    1. Get a right side R80 head and put my bike back together.
    2. Take the top end off of my parts bike R100GS and put that top end on my R80, along with the larger Bing carbs. (that was the plan, but now I hate to disassemble a good R100 motor in a rough, but complete R100GS)
    3. Keep the parts bike as is for a potential project down the road and buy used R100 pistons, cylinders and heads but use my 32mm R80 Bings.

    I guess what I'm asking indirectly is do I want my R80 to be 1000cc and if so, do I use option 2 or 3 above?

    Thanks for any advice/input.

    Attached Files:

    #19
  20. Larry Landes

    Larry Landes Adventure Rider Dude

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    Here's the parts bike I've never touched. The Red Baron.

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    #20