Squish band 430

[Wildebeest90210]

Some clarification please folks. I am putting together an 82 430 motor with a flat top Wiseco piston and am measuring 2mm/80thou squish clearance on a dummy assembly (repeated to confirm).
I'm thinking it should be 1mm but before I do any machining could I draw on the experience available here ?

 

[Bengt Husky]

I've got the workshop manual at work, it should tell you in the there. I'll have a look tomorrow and see.

 

[GaryM]

Some clarification please folks. I am putting together an 82 430 motor with a flat top Wiseco piston and am measuring 2mm/80thou squish clearance on a dummy assembly (repeated to confirm).
I'm thinking it should be 1mm but before I do any machining could I draw on the experience available here ?

1mm is the minimum you should have. See the Husky tech bulletin. Also add a hard washer on the smaller head bolts - many of the cylinders had shallow tapped holes for these I have found two cylinders with shallow holes.

 

[fran...k.]

You might read through this thread.
http://www.cafehusky.com/threads/430-500-base-gaskets.6495/#post-74589

 

[pcnsd]

The manufacturing tolerances at Husqvarna and most other engine manufacturers except the Japanese were quite poor in the 80s in comparison to today and the danger of tolerance stacking in production was addressed with different cylinder base gasket thicknesses. The danger is if you reduce the squish area by any way other than gasket selection you will run the risk of being too tight at the next assembly should you replace a rod, crank, piston, barrel or head (tolerance stacking). That said, the advantage of squish at or near the minimum is greater tolerance of lower octane fuel ratings and higher flame propagation rates (more power in the mid-range) You can not mod the head to reduce squish because it will drive the compression ratio too high, you need to turn it off the cylinder. Raising the piston in the bore is quite different from reducing CC volume.
Two-Stroke Performance Tuning by A. Graham Bell, suggest a minimum of squish clearance of .9 -1.2mm for 300-500cc motors and further suggest that you don't want to be outside that minimum.

Regards,
Paul

 

[husqyhamm]

I first get the piston to bdc, then align bottom of transfer ports to be perfectly level with piston crown for optimum flow and port timing, via base gasket thickness and torqueing barrel down to squash gasket. Thereafter assemble with head and measure squish distance.
2mm is fine. Makes for a comfortable compression, easy starts and revs quick with smooth power. Yes ive built LC engines with 1.2mm squish though things turn violent and at about 1mm squish you smack the crown of piston against head pinching ring when you overrev or slip a gear.

 

[Wildebeest90210]

Great stuff thanks. I will check the compression ratio today and tranfer port/piston relationshipand go from there. Maybe I can take a little off the head without having to machine the combustion chamber to readjust the comp ratio or maybe come up with a thinner base gasket. I'm thinking though if C ratio/ports both look OK then leave well alone. Im not after absolute power just a good solid efficient motor.
Any idea what the standard (non corrected) compression ratio should be give or take?

 

[Bengt Husky]

I've checked my manual and it doesn't give a figure...sorry

 

[Wildebeest90210]

I first get the piston to bdc, then align bottom of transfer ports to be perfectly level with piston crown for optimum flow and port timing, via base gasket thickness and torqueing barrel down to squash gasket. Thereafter assemble with head and measure squish distance.
2mm is fine. Makes for a comfortable compression, easy starts and revs quick with smooth power. Yes ive built LC engines with 1.2mm squish though things turn violent and at about 1mm squish you smack the crown of piston against head pinching ring when you overrev or slip a gear.

So, I got swept volume of 439.17cc and a combustion chamber including squish of 42.3cc. This gives 11.38:1 compression. That to me is as high as it should get, my 370 stormer was the same and just manageable/reliable.
The piston crown is spot on with the transfers so any squish adjustment would have to come off the head and then the combustion chamber machined to get the compression down again. Given the offset nature of the combustion chamber and the amount of hassle/risk getting it machined it ain't gonna happen and the 2mm squish stays as advised, thanks for the voice of reason.

 

[husqyhamm]

Swept volume over combustion chamber volume applies to fourstrokes with inlet valves fully closed on bdc. Remember your effective volume only starts when ring closes all ports and whats in there at that point in time depends on pipe sonic backpressure. With that setup youd probably run effective compression of 7,5;1. Dont worry, stick to 2mm squish and if at sometime you want to reduce to 1,5mm it will still be ok.

 

[Wildebeest90210]

Swept volume over combustion chamber volume applies to fourstrokes with inlet valves fully closed on bdc. Remember your effective volume only starts when ring closes all ports and whats in there at that point in time depends on pipe sonic backpressure. With that setup youd probably run effective compression of 7,5;1. Dont worry, stick to 2mm squish and if at sometime you want to reduce to 1,5mm it will still be ok.

Yes I got that, I go on non corrected compression as it gives me a number I understand. English 2 strokes were always measured like that then the Japanese started complicating things by measuring actual compression, typical!