SOHC.co.uk Forums > CB500/550

Pistons

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Nurse Julie:
Hi Murf.
Just send me a PM about any parts you need.
Many thanks

K2-K6:
Obviously no options with std piston to change compression specifications.

It is though a consideration, in that a camshaft with extended overlap /duration will lower the effective compression ratio, all things being equal.

The geometric ratio of combustion chamber volume to swept volume remains the same, but with valve opening moved then the active ratio will usually be reduced (assuming longer opening etc) which will reduce the torque at lower rpm.

The changes effectively push the torque peak upward to occur a raised rpm point but looses combustion effectiveness at the lower speeds.
It gives something of a " false" gain, in that the brake horse power is a calculation involving rpm as a factor in projecting work rate.

Net effect is that you have to rev it to get to torque peak by using lowered gearing, seems more dramatic peak as the reduced torque in low rpm range has greater contrast to the high rev performance.

Itll be faster by the effect of fitting in more power stroke to a measured distance that before (lower gearing, higher revs) but at the same low range rpm it will be correspondingly slower from less real torque. In other words, you've always got to ride it at revs to see an increase in power.

Lifting the static compression ratio is a way of compensation to avoid some of this.

You'd probably need a dry build though to assess piston clearance to head and that to valves at the cam duration specification used.

Murf:
Hi
Thanks for the info re F model pistons, I have ordered some pistons from IMD and as you say only flat top pistons for the 550. It has cost around £100 more than just getting new rings for the old pistons, size wise the original pistons were ok but the crowns were just so scored by the crazed wire brusher that got in the engine before me. The rings were getting close to the max gap. The standard bore is amazingly good though with virtually no wear.  I spoke to IMD and they were very helpful.  I know I am taking a risk putting new pistons in an old bore, have to hope that IMD pistons are accurate with there sizes. I will measure them when they come and pray.  I have honed  the bore to deglaze it.
I tend to agree with what is said about camshafts and an improved cam not being much good without other engine mods and I definitely don’t want an engine that is really cammy. The thing is that my camshaft was knackered, I looked around at used cams and came the conclusion that they were all just as knackered or worse than the one I had.  I decided to try the option of getting my cam repaired and reground and went for a slightly different profile.  Will it be good or not, I don’t know but thought it was worth a try.  If it’s not great then I will get another used cam, get it repaired and have it reground standard.  I got the info about Newman Cams from the forum so thanks, they are very helpful on the phone. I will give info on them when I get the cam back.

Bryanj:
IMD are very accurate with their sizing, i hve had several sets from them, honing, whilst the necessary thing, does remove metal and can put the bore out of spec, it depends on the type of hone used the nicrohone or dingleball is the best for this job

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