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How does one identify the 9.5 R100 pistons vs the standard US type lower compression versions.
Say...if a bike was represented as having the 9.5's...how would I know when I pulled the top end?
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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.
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.