Fitting 1070 kit to '77 RS

Discussion in 'Airheads' started by chasbmw, Jan 30, 2013.

  1. chasbmw

    chasbmw Long timer

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    I am asking this question on behalf of someone else.

    They have a 77 big valve RS.

    They want to fit a Seibenrock 1070 kit to the engine and have asked if this is a plug and play job

    They were advised by a US based Guru that this would not work unless the squish band cylinder head form the 77 was
    Modified to accommodate the piston shape of the Seibenrock kit.

    Can anyone here had any experiance of fitting the kit to the 77 heads? And are those heads significantly different than the heads for the later non squish band engines.

    My advice is to build the engine and check the squish and clearance, which should be?

    Thanks
    #1
  2. bmwrench

    bmwrench Long timer

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    I have not even seen the 1070 kit, except in the fuzzy pictures on Siebenrock's website. I think there are two possibilities: Siebenrock may have left enough room at the top of the piston to clear either factory squish band so that contact is avoided, or they may have set it up to work with only the late squish band. Either way, it would be wise to check clearances. Since I believe in squish, I would have the heads machined for .030" clearance. It's not expensive, and I wouldn't build a motor without at least having a look at the valves, guides and seats.
    #2
  3. Airhead Wrangler

    Airhead Wrangler Long timer

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    I'd ask this question directly to Siebenrock. I'd guess they know better than anyone else.
    #3
  4. RGregor

    RGregor Been here awhile

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    Hello Charles,

    from all I know the early heads have a radius from the gasket surface to the combustion chamber, the later ones have a 1mm step and an 18° taper.

    When installing my 1070 kit I measured the profile between heads and pistons: the SR pistons also have an 18° taper, clearance is here ~2,5mm between heads an piston (in the taper area).
    So I would say it will NOT work without machining the heads.

    Better solution IMO: buy a set of R100GS or R100R small port heads, convert them for the 44mm inlet valve.
    Sell the 77 heads.
    #4
  5. Beater

    Beater The Bavarian Butcher

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    Just so you know ... the '77 RS was one of two years (77 and 78) that had the high compression and large port set up by the factory. That engine was (and still is) the most powerful Airhead engine ever produced by BMW. By changing to the bigger displacement Seibenrock kit, you are not really increasing the power of the engine ...

    Just sayin. I'd keep this one stock.
    #5
  6. bmwrench

    bmwrench Long timer

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    Yes, but torque will be increased, and if you add a sport cam, the bike will really come alive. The next thing you'll need to do is improve the clutch.
    #6
  7. chasbmw

    chasbmw Long timer

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    Beater,

    I think you have a slightly exaggerated view of the high performance RS of the 77 era, on a good day that engine might have made 55 BHP at the rear wheel, BMWs engine bhp figures, were taken from the Crank, with an engine with anything that might have absorbed any power removed. management then added 10% for luck:clap.

    High compression big valve engines continued to be supplied to Europe until much later and I know that the 1070 kit works very well to improve those bikes. The kit also includes a new sports camshaft.

    Rudi, thanks for the advice, I will pass the message on to the owner of the bike, who is based in Australia and it seems that the guy who is doing the work to the engine might be getting out of his depth.
    #7
  8. supershaft

    supershaft because I can

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    I disagree. :D Putting in a '81 on clutch is the FIRST thing he needs to do. Then I would re-think my plans and stick with 94mm bores. I would still run those longer rods that come with a 1070 kit but with some 94mm pistons probably sold so that you can shorten the cylinders but I wouldn't shorten the cylinders so I could run those longer rods with the stock length cylinders and 94mm bores. I have seen that setup WORK!
    #8
  9. alex117

    alex117 Adventurer

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    I have a motor built in the UK 1040 cc 97 mm pistons I think J&n, 336 cam later clutch and it just fly's. It has the big valve head and seems to work really well


    I already have a nos set of 1977 big port big valve heads. I will use a later 1980's short block for the bottom end. I want to lighten the valve train maybe lighter valve spring retainers and adjusting screw to use with this 1070 kit. Seibenrock tells me it will work fine. Just trying to find someone who has done this combo before.
    Alex from down under
    #9
  10. chasbmw

    chasbmw Long timer

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    Alex,

    I hope that this thread is helpful, who built your other engine?

    I get the impression that you might have to machine the heads to get a good tight squish band which is one of the secrets to making these bikes work.

    Motoren Israel sell some nifty looking part titanium cam followers, not sure how long they might last!
    #10
  11. gsd4me

    gsd4me 90% bluff

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    There's people here in Oz that could answer his questions without him needing to go half way around the world; what city does he live in?

    If he's got someone stuffing around with his engine who doesn't know what he's doing, it's going to get very expensive.
    #11
  12. shel-bou

    shel-bou Adventurer

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    I own a 1977 RS which is running the later oversized BMW pistons without issues, so I would think the 1070 pistons with the same dome shape would be the same. But I think this is not the answer I like Rgregor idea considering the price of the 1070 kit a bit more effort would let it run as it should. Moteren Israel also sells some sport pistons -7.5mm the porfile of the pistons look like they would be a good match for the 77 heads.
    #12
  13. supershaft

    supershaft because I can

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    Titanium lifters? The biggest bang for the buck in using titanium is reducing valve float rpm. I use titanium valve spring retainers. I agree with most tuners that they do increase your valve float rpm by around 500rpm. Most tuners have agreed for 50 years now that just grams on the valve side of the rocker arm amounts to hundreds of rpm concerning valve float rpm. On the lifter side of the rocker arm? Most tuners have found that a lot of weight makes very little difference. I just TRIPLED the weight on my pushrods for more pushrod rigidity. I think it lowered my valve float rpm a couple hundred rpm at the most. I have seen modified and aftermarket lifters break. Personally, I am sticking with stock since the payoff of lighter lifters is so small and the risk of running MI titanium lifters is not known to me. Of course, most all this concern is unwarranted if you never rev your engine till the valves float anyway although a higher float rpm can help when you miss a shift! Or in my case when I broke my tire loose at around 40mph on some damp road and bounced my tach hard off of 8k rpm before I had the chance to get back out of the throttle.

    If I were going to use more titanium in my valve train, it would be the valves themselves. That will effect valve float many times over titanium lifters.
    #13