difference in piston height

[Jotabene]

This time I have an issue with my son's 750 SF -72. There was a tapping noise from the RHS cylinder. I checked the compression 9.2 Kg/cm2 vs 11.8 kg/cm2 on LHS cylinder. After dismounting the head and cylinder I found that the RHS piston ring was broken, but no damages in the cylinder bore. I have replaced one of the pistons with a NOS. It was not the same make, but I had the two pistons (the one I continued to use and the NOS) balanced and the cylinders honed by a professional motor mechanics. After some 1500km I checked the compression and found that the compression is still low on the RHS compared to LHS 10.1 vs 12.1. There are still some tapping noise but not as pronounced as before.
I dismounted the head an cylinder again and found out the distance between gudgeon pin bore and the flat surface on the top of the piston (closest to the edge i.e not top of the bulge) differ something like 0.55mm.

What's you opinion can this difference cause a difference in compression of 2 kg/cm2?

I Would appreciate advice or comments.

 

[Laverda SF]

Does sound like you have a Compression Height issue with different pistons.

This PC App may be of benefit: http://cochise.uia.net/pkelley2/DynamicCR.html Although it the App doesn't really account for Piston Compression Height but does account for all other variables.

 

[IAG]

Yes, matched pistons only-when the mechanic installed the replacement piston was piston o.d. and cylinder I.d. measured? Is piston slap a possibility? When the head was removed was there any evidence of a valve contacting the piston & finally what were valve clearances set at?

 

[motoddrob]

Ticking noise could be the cam halfs moving independently to each other. Check the woodruff key between the two.
I would never think of using different pistons in a motor either.

 

[Jotabene]

Thanks for all inputs.
Just to clarify, the shape of the top of the pistons i.e. dome is identical in shape and height. No sign of touching the valves. Valves are not leaking.

In any case I will most probably get a new pair of pistons.

 

[Tippie]

Your cylinder compression tests do not necessarily reflect compression ratio and the procedure is different. Different heights will give different compression ratios, and sealing issues will give different compression test figures. Isn?t there a certain percentage allowance for compression tests? You are using unusual figures in your kg/cm2, generally we use psi, hence the confusion from Roger. If you are playing with compression ratio it is vital that you measure clearance at both squish band and valves for 1.0mm clearance at TDC and through overlap. In any case the volume of the chambers should be as close as possible on each side and from the sound of it you have an issue there. If you are simply checking compression ratio you can follow the steps with warm engine, wot, oil in bore to isolate ring leak etc. The best for that is a leakdown test.

 

[piranha-bro2]

If you are simply checking compression ratio you can follow the steps with warm engine, wot, oil in bore to isolate ring leak etc. The best for that is a leakdown test.

I think that should read 'checking compression' ... 

If you want an accurate compression ratio measurement, you need a burette or similarly accurate volumetric measuring device. The correct way to calculate ratio is to add the combustion chamber volume (piston at tdc, fill with oil-kero mix to halfway up the plug hole) to the swept volume (cc of the cylinder = area x stroke) and then divide by the combustion chamber volume. The figures will be lower than you expect. My stock SF1 read about 8.9:1 from memory. If you simply divide combustion swept volume by chamber volume you will get an inflated reading. 12:1 is VERY high.

Apart from all this, a ticking sound from the cylinder doesn't sound to me like a piston issue; a healthy motor doesn't just start making ticking noises from the piston - more likely in the valve train, maybe side play in a rocker, a loose tappet (see threads on pitted SF valve stems and subsequent excessive valve clearances) or as Motoddrob says, loose cam flange/s.

Uneven compression readings are MUCH more likely to be due to leaking valve seats or piston ring leakdown.

And I'd reiterate that fitting a non-eq piston is an invitation to problems.

My 2-bob's worth ...