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Identify high compression pistons?
- Thread starter syvrider
- Start date
pglaves
#13338
I don't know, but I would start with a compression test.
Did that. Near zero leakdown and compression around 160 each side.
So they are almost certainly the 9.5's. Dual plugged heads.
But I've got another project going and some pistons that came with some other parts that are allegedly the 9.5s. Was trying to find out for sure if that's true.
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So they are almost certainly the 9.5's. Dual plugged heads.
But I've got another project going and some pistons that came with some other parts that are allegedly the 9.5s. Was trying to find out for sure if that's true.
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kinterridge
Member
Old thread by AnnaplisAihead in 2011, 04-09-2011, 08:51 AM labeled Piston Identification shows the difference the area above the rings is the telltale sign, it is taller. He's got pictures.
Here is AnnapolisAirhead's post:
https://forums.bmwmoa.org/showthread.php?52359-Piston-Identification
https://forums.bmwmoa.org/showthread.php?52359-Piston-Identification
Thanks for that. The pistons I have on the shelf have 5 grooves, not 7. So maybe they're not 9.5.
At some point I'll be doing a top end refresh on my GS with the dual plugs and high compression numbers and I can look at those pistons. But with nearly zero leakdown and no oil leaks, I'm inclined to set curiosity aside and leave it alone.
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At some point I'll be doing a top end refresh on my GS with the dual plugs and high compression numbers and I can look at those pistons. But with nearly zero leakdown and no oil leaks, I'm inclined to set curiosity aside and leave it alone.
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I found something Brook Reams posted in the Airheads mail list about 18 months ago. He wrote that a high compression piston will have a dome between the valve pockets while the lower compression piston has a flat top.
There is another change that had to do with the squish band at the outer edge on top of the pistons. I think in the '77s, there was a flat area out near the edge that was supposed to create turbulence and help with combustion. The head had matching area. But I think they found somewhat incomplete combustion with that and the flame front didn't do a good job of burning all the mixture. Later in 1977 into 1978, the piston dome was smooth/round shaped all the way to the edge...head matched that.
There is another change that had to do with the squish band at the outer edge on top of the pistons. I think in the '77s, there was a flat area out near the edge that was supposed to create turbulence and help with combustion. The head had matching area. But I think they found somewhat incomplete combustion with that and the flame front didn't do a good job of burning all the mixture. Later in 1977 into 1978, the piston dome was smooth/round shaped all the way to the edge...head matched that.
pglaves
#13338
That was to get the engine to comply with the then brand new 1978 emissions standards.
I scrolled through a bunch of eBay listings for pistons. While you never know if the seller is properly listing the year/model, but it sure seems all over the place. Domed pistons on later model 8.2:1 bikes. Older ones that *should* be higher comp with 5 grooves and also with 7 grooves. Flat tops. Domed to the edge. Domed part way to the edge.
It's still a mystery to me.
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It's still a mystery to me.
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lkchris
Active member
This discussion has to start with 1981 bikes, as USA bikes before that had 9.5 pistons if they were S or RS or RT and had 9.0 if they were /7. This is for 100s.
There was a period of time in the latter half of the 1970s that premium fuel was unavailable in the USA or at least difficult to find due to the transition to unleaded fuel. Gas stations didn't have enough pumps, and used their former pumps for premium for the new unleaded, which was always low test. Most 'merikun cars ran on regular anyway, but this was a problem for BMW motorcycle owners.
The original idea to dual plug Airheads was an attempt to get them to survive running their usual high compression but with regular gas. This was the ONLY reason for it, but over the years "legend" arose that somehow this enhanced the performance of the bikes. These things are of course a self-fulfilling prophecy, as anyone that paid for dual plugging of course felt a performance increase. Don't think it was there in actuality.
So, it was pretty easy to convert your low compression '81-on to 9.5 pistons ... just had to be nikasil compatible pistons and the correct size code ... A, B, or C. Most were B. A look at the Bing book for these 100s indicated the same carb specs except for one notch higher for the needle jet. Yes, a little bit richer ... and of course you now ran premium. I think there should have been a timing difference between a low and a high compression motor, but there was none for the '81 on bikes--the hemi combustion chamber is not a particularly sophisticated solution in any event. Fuel economy should improve with higher compression, but I never compared mine before and after. Into riding, not record keeping.
There was a period of time in the latter half of the 1970s that premium fuel was unavailable in the USA or at least difficult to find due to the transition to unleaded fuel. Gas stations didn't have enough pumps, and used their former pumps for premium for the new unleaded, which was always low test. Most 'merikun cars ran on regular anyway, but this was a problem for BMW motorcycle owners.
The original idea to dual plug Airheads was an attempt to get them to survive running their usual high compression but with regular gas. This was the ONLY reason for it, but over the years "legend" arose that somehow this enhanced the performance of the bikes. These things are of course a self-fulfilling prophecy, as anyone that paid for dual plugging of course felt a performance increase. Don't think it was there in actuality.
So, it was pretty easy to convert your low compression '81-on to 9.5 pistons ... just had to be nikasil compatible pistons and the correct size code ... A, B, or C. Most were B. A look at the Bing book for these 100s indicated the same carb specs except for one notch higher for the needle jet. Yes, a little bit richer ... and of course you now ran premium. I think there should have been a timing difference between a low and a high compression motor, but there was none for the '81 on bikes--the hemi combustion chamber is not a particularly sophisticated solution in any event. Fuel economy should improve with higher compression, but I never compared mine before and after. Into riding, not record keeping.
Good info, Kent, thanks.
As I recall, another popular mod was the aluminum cylinder base shims, that effectively LOWERED compression enough to avoid the worst of the pinging. Even my old and slow R60 would ping on the gas of the day, especially the left side after it had carboned up a bit from side stand use. I think the later, lower compression pistons were a good thing overall.
The early series R60 engines had high compression and a mild cam which resulted in a tendency to ping. The ignition had to be spot on...the electronic ignitions like the Boyer which delayed the onset of advance helped with that a lot. There is a part number for the compression-lowering gasket for the R60...I am using it on my R100/7 top end because the Siebenrock kit had compression above 9.0 which I wanted to avoid.
