Author Topic: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp  (Read 2591 times)

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #40 on: Oct 09, 2017, 14:45:24 »
Over the weekend I managed to start the tear down of the top end and do a bit of a measure up.  I wanted to know the exact weight of the piston assembly so we have a baseline for the new big bore piston.  I hope that with new methods, we can achieve a bigger piston with the same weight as this one so that the balance of the engine (both the primary balance for the crankshaft and the secondary from the balancer shaft) remains the same - so not to effect the vibration levels.  If this cannot be achieved, we will taper bore the piston wrist pin to get the weight back to stock.

Then the piston producer wanted to know valve angle and wanted me to mark the valve centres on the piston so he has a reference for machining out the valve clearance pockets.  The valve angles in the photos don't show up very accurate because of the perspective, but they were both 115 degrees.

I also measured the compressed height of a used head gasket and the deck height.  Deck height seems to be zero...  So just relies on valve to piston clearance and the clearance from the head gasket height.  Eventually I will cc the head and add this to the volume of the compressed gasket.  Then I will be able to get an exact compression ratio and be able to calculate and predict a new compression ratio with the new piston and the new swept volume.

The head volume is listed in the manual as 30.4cm3, head gasket thickness as 1mm (which is correct for a compressed one) and compression ratio as 8.9:1 (for the 239cc versions) and I have no reason not to believe this, but I want to measure it anyway.  I will also use that compression ratio and these listed numbers to reverse calculate the head and gasket volume just to double check my numbers and see if there are any differences.