Specific questions - 800cc to 1,000 cc

Discussion in 'Airheads' started by Bluethumb, Oct 10, 2012.

  1. Bluethumb

    Bluethumb Been here awhile

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    I'm looking to add a set of 1,000 cc cylinders to my R80ST, keeping my original heads and carbs from the R80. From what limited knowledge I have I believe that this is a straight bolt on upgrade. Remove the 800 cc cylinders and pistons and install the 1,000 cc cylinders and pistons. Am I correct on this? Will the 800 cc heads bolt right on? What head gaskets do I order? And do I need to make any changes to the carbs? Different position on the jet needle, different needle jet, etc.

    From what I've read, this is the one upgrade that basically gives the most bang for the buck. I believe I'll get a decent boost in torque and h.p., and still be able to run regular gas. I also believe I've read that 800 cc heads and 32 mm carbs bolt on with no problems.

    The cylinders and pistons are from a '81 R100RT so I m assuming the pistons are 8.2 to 1.

    Am I basically good to go in terms of this being a "bolt on" operation or is it much more involved then that?

    Thanks all
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  2. Airhead Wrangler

    Airhead Wrangler Long timer

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    You should have no problems with that setup. You'll want the 1000cc head gaskets though, not the 800cc ones. Ideally you should have the combustion chamber reshaped to the larger piston, but it will work as-is. I'd assemble it and check piston/head clearance with some solder before running it, but that's just to be safe. You shouldn't have any interference if they are in fact 8.2:1 pistons. I haven't heard accounts of using the higher compression pistons with 800cc heads, so I'm not sure how that would work out.
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  3. Biebs

    Biebs BMW Airhead

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    Sounds like a simple 200 CC upgrade. Please repost the results:1drink

    don't keep us waiting to log a fellow could drink to much!!!:freaky

    Also this is a great Ride Report from Airhead Wrangler:

    http://www.advrider.com/forums/showthread.php?t=530663

    Just read it through great job - speaking of jobs how is the job market in Mexico??
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  4. tdrrally

    tdrrally Long timer

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    :lurk
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  5. supershaft

    supershaft because I can

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    I have heard it works. I have done the swap many times but I always machined the combustion chamber to match the cylinder. If you want to raise the compression beyond 8.5:1, I think you have to. I think the 8.2:1 pistons are 8.5:1 pistons with the tops lobbed off. The stock 3.36:1 FD ratio seems mighty short with the extra 200cc's. Personally, I think a 3.2:1 would be about perfect.
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  6. Airhead Wrangler

    Airhead Wrangler Long timer

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    Great if you like working 6 day weeks. Most down here do. The trick is finding an American company down here that has a weird need that can only be filled by an American. They pay well for that because Mexico is so "dangerous" according to fox news. :lol3
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  7. Bluethumb

    Bluethumb Been here awhile

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    How are the combustion chambers machined out and by who? I'm looking for an inexpensive upgrade. Not looking to do an all out hot rod motor. 1,000 cc cylinders and pistons come up regularly on the marketplace; I've got a line on a set right now. I could also pick up the 1,000 heads but then the project starts costing serious money. Cylinders and pistons can be had for $300-400. Heads can double that. Then you're looking at 40 mm carbs for $250-300 and suddenly a project with gaskets, pushrod seals that costs $400 max suddenly becomes a $1,200 plus project.

    I thought I've seen others w/ 800 cc motors, usually R80G/S's, upgrade by only swapping out the cylinders.
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  8. Bluethumb

    Bluethumb Been here awhile

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    And what's the benefit of maching the ombustion chamber? Is it only to make sure there is clearance?
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  9. Airhead Wrangler

    Airhead Wrangler Long timer

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    Usually those are Siebenrock kits. They make 1000cc pistons that match the combustion chamber of the 800cc heads. I'm not sure how much "better" that makes them, but they also have a higher 9.5:1 compression ratio.
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  10. Airhead Wrangler

    Airhead Wrangler Long timer

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    You get rid of the sharp edge on the combustion chamber which can be a hot spot and cause pinging on higher performance engines. Better flame spread, more of a squish band, etc. For your purposes, I think you'll be perfectly happy not going to the extra trouble. If you wanted to hot it up a bit more though with higher compression pistons you'd need to do that to avoid interference.
    #10
  11. bmwrench

    bmwrench Long timer

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    I have done several 1000cc conversions using OEM cylinders and 8.2:1 pistons to R80s with good results. At that time, you could buy a piston/cylinder assembly for under 400.00 (!) One of them has now covered over 100,000 miles. This bike, an '87 R80, has a 3.2:1 Rear Drive ratio and has spent a good deal of time running at 80-90 MPH. A couple of years ago, we decided to machine the heads for a better match and tighter squish clearance. This made a slight increase in performance and fuel consumption. It will pul l past redline in top gear.
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  12. supershaft

    supershaft because I can

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    What AW said. You don't need to if you are not raising compression. Put a head on one of your 800cc cylinders and look up the bore and then do the same thing with the 1000cc cylinder. You'll see what we are talking about. Squish band? If you don't raise compression machining your squish band to match the piston will reduce your squish, not increase it.
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  13. Airhead Wrangler

    Airhead Wrangler Long timer

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    Yeah, I wasn't very clear on that. If you want a squish band, you first need to mill away the hard corner that R80 heads create in R100 barrels in order to better match the periphery of the piston. Then plane down the head (or barrel) to set desired squish. No, milling out the combustion chamber in and of itself will not give you a tighter squish band... quite the opposite.
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  14. Bluethumb

    Bluethumb Been here awhile

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    OK, now I'm confused! Swap out the 800cc pistons and cylinders with the 1,000 cc pistons and cylinders, and bolt on the stock 800 heads? I need to do nothing with the heads, correct?

    And would somebody define squash band for me? In my mind, I see it at the space created when the piston is at TDC, the space between the piston top or dome and the curved surface of the cylinder head. Is this basically what a squash band is?
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  15. bmwrench

    bmwrench Long timer

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    The R75-R90 heads have a flat band around the outside of the chamber and a matching one in the piston. The piston comes closer to this band than the hemispherical part of the chamber. This close clearance causes turbulence in the chamber, mixing the fuel and air better than if there were none. I don't know when or who first called it a squish band-I suspect the British-but it has been called that as long as I can remember.
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  16. supershaft

    supershaft because I can

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    All the BMW's heads/pistons have a squish band. The earlier ones are square with the bore and the later ones are angled. The squish band is the flat or angled area of the head before it dips deeper into the more hemispherical portion of the compustion chamber. It will all become clear when you put one of your heads on both size cylinders and look at the combustion chamber through the bottom of the cylinder. A piston's view of the situation so to speak.
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  17. R100RT Mark

    R100RT Mark Been here awhile

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    So, for the sake of clarity, and my education, what are the specific differences between U.S. late model 800cc and 1,000cc heads?
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  18. Airhead Wrangler

    Airhead Wrangler Long timer

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    From '85 on, I believe it's just combustion chamber diameter. Depth is the same, I think. Valve sizes are the same.

    EDIT: I'm a visual person. This is what a section of the two heads superimposed on each other would look like:

    [​IMG]

    Dark grey is the R100 head. Light grey is the cylinder walls. The red is the extra material present on r80 heads. The bore difference is shown to scale. The chamber depth and profiles of the squish band areas are not, but are close enough to show the general differences.
    #18
  19. R100RT Mark

    R100RT Mark Been here awhile

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    So, castings the same, just machined differently. Yes? Valve seats in the same position?
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  20. supershaft

    supershaft because I can

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    Yes and yes.
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