Hi friends, I hope to find advice from our knowledge pool.
I started to evaluate the required thickness of my cylinder spacers and base gasket in order to get the desired squish of 0,70mm. I put a spacer with known thickness and a 0.3mm paper gasket on the engine cases, bolted the cylinder down with the correct torque. Then I laid 2 pieces of 1mm solder on the piston top, 90deg to each other so that I could use the average of the 4 measurements for each end for the correct squish. Bolted down the head (O-ringed, no head gasket), again with the correct torque.
The measurement with this 1mm solder resulted in a 0,62mm squish.
Then I did the exact same procedure, using a 2mm solder instead of the 1mm. I noticed that turning the crank over TDC now required much more torque, due to the thicker solder. The reading of the measurement showed 0,75mm now! With the thinner solder, there was very little resistance when turning the crank.
My conclusion is that the difference of 0,13mm must be the sum of the clearances of both lower and upper conrod bearings, but this seems too much for me.
- has anyone experienced this difference?
- which measurement should I work from?
Squish Measurement
Moderator: rztom
Re: Squish Measurement
Even plastic deformation has an elastic component.
My theory would be that the thicker material may "spring back" some 1/100 mm.
Plus the higher force required to deform the thicker material would "deform" the affected parts (piston, conrod, roller bearings,...) which will add some deformation, too (actually it's like a serial connection of springs).
Assume a bearing stiffness of 25 kN/mm and 2 kN required to quench the solder wires, then you get around 8/1000 mm of deformation.
(both values guessducated)
You could reduce that effect by turning the crank more than once (or forward / rearward for a couple of times) or by using a softer compound like play dough.
My theory would be that the thicker material may "spring back" some 1/100 mm.
Plus the higher force required to deform the thicker material would "deform" the affected parts (piston, conrod, roller bearings,...) which will add some deformation, too (actually it's like a serial connection of springs).
Assume a bearing stiffness of 25 kN/mm and 2 kN required to quench the solder wires, then you get around 8/1000 mm of deformation.
(both values guessducated)
You could reduce that effect by turning the crank more than once (or forward / rearward for a couple of times) or by using a softer compound like play dough.
Bye
Martin
Martin
Re: Squish Measurement
Makes sense, Martin. But it should read 8/100mm, which comes close to my findings. How do you typically measure your squish ( which size of solder, 1 turn or more?)
Re: Squish Measurement
I usually target some 0.9 mm for my 350 and have around 2mm soldering wire.
Can't tell if I rotate once or multiple times as it's long ago I needed to do that....
On the other hand I'm not so keen on using the last 1/100 mm possible.
BTW: That mentioned elasticity also applies when the piston is at TDC at top rpm. In that case the crank webs bend quite a lot and that's why you need a certain squish at all.
On the 350 a value 0.6 mm was too low for example and the piston had contact with the head.
Modern software can predict that flexibility and would help you to get in the ballpark if you're a motorcycle OEM.
https://youtu.be/OE66R3K4VB8
Can't tell if I rotate once or multiple times as it's long ago I needed to do that....
On the other hand I'm not so keen on using the last 1/100 mm possible.
BTW: That mentioned elasticity also applies when the piston is at TDC at top rpm. In that case the crank webs bend quite a lot and that's why you need a certain squish at all.
On the 350 a value 0.6 mm was too low for example and the piston had contact with the head.
Modern software can predict that flexibility and would help you to get in the ballpark if you're a motorcycle OEM.
https://youtu.be/OE66R3K4VB8
Bye
Martin
Martin