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Old 10-05-2010, 10:30 AM   #1
Rob Farmer OP
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Anybody clued up on airhead pistons?

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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Old 10-05-2010, 11:26 AM   #2
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Quote:
Originally Posted by Rob Farmer
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?
I believe squish-band pistons are 1977 only.
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Old 10-05-2010, 01:38 PM   #3
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Lornce always has something to say about squish band pistons. Maybe he has some insights.
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Old 10-05-2010, 01:40 PM   #4
Rob Farmer OP
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Not sure about the pistons you had in the states but mine are from a UK 78 -they're the same as this fellas - http://moragafalconers.org/bmw_pisto...92%20sides.htm
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Old 10-05-2010, 01:42 PM   #5
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Quote:
Originally Posted by fishkens
Lornce always has something to say about squish band pistons. Maybe he has some insights.
Lets hope so
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Old 10-05-2010, 02:48 PM   #6
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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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Old 10-05-2010, 06:36 PM   #7
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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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Old 10-05-2010, 07:30 PM   #8
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Quote:
Originally Posted by bmwrench
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.
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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Old 10-05-2010, 09:57 PM   #9
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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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Old 10-06-2010, 05:33 AM   #10
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Quote:
Originally Posted by spartanman
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.
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.
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Old 10-06-2010, 09:10 AM   #11
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The '70s pistons are for iron cylinders and the '81 cylinders are nikasil.
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Old 10-06-2010, 11:00 AM   #12
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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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Old 10-06-2010, 11:01 AM   #13
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Quote:
Originally Posted by lkchris
The '70s pistons are for iron cylinders and the '81 cylinders are nikasil.
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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Old 10-06-2010, 04:00 PM   #14
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Quote:
Originally Posted by Rob Farmer
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?
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.

supershaft screwed with this post 10-06-2010 at 04:06 PM
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Old 10-06-2010, 10:41 PM   #15
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Quote:
Originally Posted by supershaft
And it is rated for more hp? Maybe it's the square airbox?
You won't let it drop will you 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
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