Anybody clued up on airhead pistons?

Discussion in 'Airheads' started by Rob Farmer, Oct 5, 2010.

  1. Rob Farmer

    Rob Farmer Long timer

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    I'm putting together a spare engine ready for a project bike. Sitting in my pile of spare parts I have a pair of 78 100RS pistons and a pair of UK spec 81 100RS. Both 9.5:1. I also have a spare pair of heads for each year to go with them.

    Apart from the 40mm exhaust ports the 78 heads are identical to the 81 heads. The UK spec 81 engines were rated at 70 BHP while the 78s were rated at 67 BHP.

    I've got enough engine, clutch and gearbox parts to have the engine as either a heavy or light flywheel. I'm leaning towards a heavy flywheel. The bottom end is an 81RS so I'll be keeping the bean can. I'll need to use the 38mm heads because the bikes GS based so I'll need the gs exhaust.

    The question is which pistons to use? Normally I'd just whack the 81 pistons in but I've seen plenty of mention of squish bands being better on the earlier pistons.

    What do you reckon guys. A worthwhile mod or just stick with the 81 pistons?
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  2. spartanman

    spartanman regret minimizer

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    I believe squish-band pistons are 1977 only.
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  3. fishkens

    fishkens Long timer

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    Lornce always has something to say about squish band pistons. Maybe he has some insights.
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  4. Rob Farmer

    Rob Farmer Long timer

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  5. Rob Farmer

    Rob Farmer Long timer

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    Lets hope so :ear
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  6. spartanman

    spartanman regret minimizer

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    From Snowbum's website...

    g. Note that high compression pistons may have problems with the head. Later pistons had the dome extending to the edge, and the later
    1970's types with the squish band were designed to work with a matching cylinder head, and might have to be machined. This might
    occur with some modification you are attempting, such as later pistons fitting 1977 heads. The squish pistons used on such as the 1977
    R100RS are hard to find. The 1977 year was different regarding the pistons. The 1978+ had a different shape for the pistons...at
    the edge as an example. The 1977 style pistons are probably no longer available. Perhaps some PRE-emissions 1978 pistons
    will work, but you would have to order them and look at them. The 1977 R100 pistons were special, only used in that year and
    model, to be a bit more precise here. If you have a 1977 R100 and need pistons, you could consider using the 9.5:1 1978
    pistons with the outside circlip.

    I've heard from a few sources '77 pistons are one-year only, which is probably why they are impossible to find. The '78 pistons apparently have a squish band as well, albeit a different shape. In any case, I'd use some putty to check piston head to cylinder clearances, especially in the squish region. Too close and you'll get contact at higher revs (not good).
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  7. bmwrench

    bmwrench Long timer

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    If my memory serves me (you know how that works), flat squishband R100 pistons were used from 77 until June '78. They unfortunately used the heavy circlip which was prone to destructive failure.

    The later pistons without a squish band do benefit from some squish when used with the appropriate heads, which have an angled squish band. A squish band does not have to be flat to work; it just has to have the edge of the piston to come close enough to promote turbulence in the chamber.

    I would use the later pistons and have the heads milled/turned so as to get the squish clearance to .030". Or if you prefer, turn the cylinder bases cut back to achieve the same.
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  8. spartanman

    spartanman regret minimizer

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    I was aware the 77s had a special circlip, but never heard they were prone to destructive failure. Any idea why? Agreed, .030-.050" squish will give the desired result without risk of piston/head collision at high engine rpm.
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  9. Rob Farmer

    Rob Farmer Long timer

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    Thanks guys. Great replies.

    My 78 pistons have internal circlips so I take it they are the proper squish band pistons.

    My 78 heads have exactly the same part number as the later heads but have 40mm exhaust ports instead of the later ones 38mm. The combustion chambers are identical on both sets of heads. I'll get some putty, put everything in place and take some measurements. First impressions are that I can simply fit the early pistons to the later heads as long as I fit later rings to go with the nikasil barrels. Do you think it's worth the effort? My cam is going to be the later rs one so the cam timing will be slightly different to the early engine.
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  10. bmwrench

    bmwrench Long timer

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    The circlips were so stiff that they tended to bend instead of springing back to their original shape. This resulted in the clip being loose in the groove, which could then be pushed out by the wrist pin.

    The quality of these circlips must have varied, since there hundreds of R100s that survived. I recently received some circlips for these pistons that work as they should, which made me very happy.
    #10
  11. lkchris

    lkchris Albuquerque

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    The '70s pistons are for iron cylinders and the '81 cylinders are nikasil.
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  12. Steptoe

    Steptoe steptoe

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    Back in the past I've had a couple of instances of late post 81 1000cc barrels and pistons fitted to a pre 81 bottom end gently tapping against the crank web ( if i remember correctly, it was a long time ago), which over a period of time cracked the pistons.
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  13. wirewrkr

    wirewrkr the thread-killer

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    The pistons themselves are not designated to the material of the cylinders, it's the rings that are different between cast iron and nikasil.
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  14. supershaft

    supershaft because I can

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    Everything is the same except the '81 has smaller exhaust pipes? And it is rated for more hp? Maybe it's the square airbox? Where did that free hp come from? At some point the '78's got advanced cam timing. I believe the '81 has advanced cam timing for sure but that should reduce peak power not increase it. It sounds like your '78 came with a squish band piston and later style heads? The whole point of having that flat squish band on the piston is so that it matches up with a flat squish band in the head. Did that mismatch get the lower hp figure? Personally, I don't have that much faith in the factories hp numbers to make any decision based on them.

    I have learned through a good friend that the angled late model heads work very well with the stock non-squish band pistons as far as creating ping free extra compression. That is what squish bands are ALL about after all.

    Personally, I think you can make MUCH more accurate piston clearance measurements with soft solder versus clay. It is so much easier to measure down to a nat's ass. It's less of a mess in a lot of ways. It's not something that you want to do over! Plus you can measure both piston and valve clearance with solder WITHOUT taking the engine apart IF you are diligent.

    I have never been around raising compression via shortening the cylinders. From the bottom sounds like it could cause push rod tube leaks to me? From the top might get you into the push rod tubes on a lot of cylinders. I could see having to move them and that effects the seal on both sides of the tube.
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  15. Rob Farmer

    Rob Farmer Long timer

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    You won't let it drop will you :D The only reason I fitted the early airbox on my PD was because the bike came with a mismatched engine, loads of dyno runs showed I had a problem with airflow at high rpm, the early airbox fixed this. When I get round to it with the exception of high comp pistons the engine will be put back to standard including the airbox. As a point of interest one of the guys over here spoke to HPN about a similar issue and was told the standard flat airbox doesn't have enough volume for the 40mm carbs, apparently if you make it 2cm deeper it makes a big difference to performance on 40mm carbed bikes. Morespeed also make a high volume replacement airbox and he's getting over 100 BHP out of his engines so there's something in it.


    I have enough parts to put a complete 81 - 84 UK spec RS motor together, I also have the 78 squish band barrels and pistons that I haven't really got a use for. The whole point of the thread was to try and figure if there was any benefit to using them. I'll stick with the standard 9.5:1 pistons.

    Good info though and thanks for the input. It is appreciated :thumb
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  16. supershaft

    supershaft because I can

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    I was just teasing about the airbox. Nevertheless, saying that the square airbox doesn't flow enough for 40mm Bings makes no sense. For instance, the most air an engine pulls is usually right at or around peak torque. Air flow and torque are very closely related. You can tune an engine with a square airbox to make a lot more torque and hp for that matter than any '77RS ever had. BTDT. No, I don't think that square airbox is the best thing on the planet but it is by far not the worse.

    Increasing airbox volume? If I had a vintage road racer and a dyno, I think a LOT of power could be had by making a large still airbox. Even GP bikes have had airboxs for some time now! I think of them as being something like an air battery. Instead of your engine having to suck in the whole world every time an intake valve opens, it only has to worry about what is in the box. Of course, that box needs to refill VERY quickly but . . . . That's how I look at that forest.

    Thanks for calling it good input. It's fun. Hopefully it can help someone. It's great to share experiences and opinions.
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  17. 2ndlaw

    2ndlaw Adventurer

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    No. Peak volumetric efficiency and peak torque generally coincide. Therefore, peak airflow per revolution and peak torque are closely related.

    Airflow is proportional to volumetric efficiency and rpm. Provided everything is jetted right, peak airflow will be near peak power.

    FWIW, I've never bought into the round air filter is better, though. It would be worth testing, but I haven't even found the time to go redyno my GS with the SR-Racing exhaust system.
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  18. supershaft

    supershaft because I can

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    You have lost me 2ndlaw. Don't you mean peak airflow PER INTAKE STROKE. When else does air flow? Does it somehow magically keep flowing all that time the intake valve is closed? What does rpm have to do with how efficient the intake stroke is? The highest volumetric efficiency IS the most air flow. Air flows one intake stroke at a time.

    We agree that peak torque, peak volumetric efficiency, and peak airflow all occur about the same time per intake stroke or "revolution". I will agree that peak airflow occurs at peak power like you said if you agree that peak torque is peak power. After all, hp is just a figment of our imagination. Surely you understand that peak torque is peak power for any engine that does not rev above 5252 rpm. HP is ALWAYS less than torque below that rpm. Peak hp and peak torque are always at exactly the same rpm below 5252rpm. Above that magical rpm, hp is always greater than torque but it is still absolutely tied to torque production and thusly airflow. Hp will continue to rise after peak torque as long as the revs are picking up faster than the torque is falling off. Peak hp isn't peak power, it's peak amounts of power in a given time frame. Us humans have made that time frame up. Peak power happened back at peak torque along with peak volumetric efficiency. In other words, when peak hp is above peak torque, it is because the engine can manage to suck less air faster for a while. How are you figuring peak air flow and peak volumetric efficiency are different?
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  19. 2ndlaw

    2ndlaw Adventurer

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    The highest airflow per stroke (unit air charge) is at the highest volumetric efficiency.

    Some math:
    Unit air charge=volumetric efficiency*displacement*density of air
    Airflow/time=unit air charge at a given speed*speed (speed meaning rpm, rps, etc.)
    intake strokes/time=engine speed/2 (for a four stroke)
    Airflow/time=speed/2*volumetric efficiency*displacement*air density

    Peak torque is around where airflow/stroke is maximized.
    Peak power is around where airflow/time is maximized.

    Here are some quotes from Obert's "IC Engines and Air Pollution",1973. Obert was probably the best engineer ever as far as explaining things in a practical matter. Much better than me!

    "For a hypothetical SI or CI engine at WOT with constant percentage heat loss, the indicated torque and the unit air charge are directly proportional at each engine speed...Practically, it can be considered that the point of maximum torque occurs at essentially the same speed as that for maximum air charge."

    "Suppose that the unit air charge at wot (in lb per stroke) were to be multiplied by the number of strokes per hour of the engine. The result would be the air consumption of the engine (My note: what we call "air flow" here.) (in lb per hour)...Here it can be realized that that both speed and unit air charge control air consumption and that the air consumption continues to climb even though the unit air charge has passed its maximum point. This increase is finally halted by a rapid fall in the unit air charge...For a hypothetical engine with constant percentage heat loss, the indicated horsepower and air consumption are directly proportional."

    Hope that helps.
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  20. 2ndlaw

    2ndlaw Adventurer

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    Horsepower is not a figment of our imagination. It is one unit for power, the rate at which useful work can be accomplished.

    Torque is a measure of force.

    In other words, torque let's us know if we can climb a hill, power let's us know how fast.

    Power=torque*angular speed. If speed increases faster than torque falls with increasing speed, power increases. 5252 has nothing to do with it.
    The 5252 is just a unit conversion factor when torque is in ft-lbs and power is in horsepower. It has no other physical significance.
    #20