Base gasket thickness?

[Husky390]

Hi,
Does anyone know the thickness of the base gasket for the 390? I am in a hurry and need to get it back together so can't wait for one to come so will make my own.
I imagine it is important for compression / piston to head clearance?
Cheers
Russ

 

[fran...k.]

If it leaks and the engine starts to run away after you pulled the plug wire off kick it into gear to stall before it destroys itself by really running away.

53 thousands if it is the same as what I measured which would be from 82 up. post 10 http://www.cafehusky.com/threads/squish-gap-1985-500xc.40202/#post-390277

 

[Husky390]

Thanks Fran...k. It is a 1979 390cc. I pulled it down to check everything and want to put the correct gasket material in.
Cheers
Russ

 

[390Dave]

As a rule of thumb,steel connecting rods will stretch .005" for every inch of length in service{youngs modulus of elasticity}so,135mm center to center connecting rod is @5.315",so with a margin of safety for thermal expansion, crankshaft deflection, bearing wear, and allowance for oil clearance,.030" is the safe minimum I would want for an engine that gets torn down frequently for inspection and bearing replacement,.035" or more but not exceeding .060" for something that is going to run until it wears out again.Good instructions on solder checking:www.klemmvintage.com 390 cylinders,even from the same model year, can vary in height by over 1mm,.0395,like anything else with production tolerances.

 

[Husky390]

Thanks 390Dave. I will check the piston to head clearance. I assume this is what you are saying and I am looking for around 0.035" for the thickness of solder?
Cheers
Russ

 

[Ron]

FYI, in a hurry I used some .060" thick gasket material for my 77 250CR. The old one measured about .040" compressed.
The motor lost a lot of compression and was very hard to start (even with a fresh piston and rings).
I switched back to the stock gasket as quick as I could and everything is back to normal.

 

[390Dave]

Russ,.035" is good,using the solder.I believe that Ron has a valid point also.The 390 in my avatar photo,I chose the "tallest"cylinder and the "deepest" chamber cyl head from parts I had,with a thick Cometic base gasket,squish ended up at .070",.060" was what I wanted,since this bike will be used mainly as a trail cruiser/dual sport,and on junk pump gas,and it does have less throttle response than before,but more top end.It starts easily and pulls strong,thanks to an MZB ignition,and advancing the timing to @2.5mm btdc.If you search the forums,subarumy98 or husqyhamm,built a low compression 500cc,with good results.The 500cc factory desert racebikes,had the compression ratio reduced to increase top speed,I dont know what method George at Uptite used,whether the cyl heads volume was increased,the piston dished,or both.If you had a 125cc,with less swept volume,and a 94 or 97mm con rod,.025" squish might be whats needed.

 

[Husky390]

Thanks Dave and Ron,
Very interesting.
I don't understand how top end increases with more squish. Is it more efficient with low grade pump fuel, revs out higher or some other reason?
I use 100 octane race leaded fuel for road racing.

 

[Ron]

"I don't understand how top end increases with more squish. Is it more efficient with low grade pump fuel, revs out higher or some other reason?"

Actually, the opposite is true. More squish helps control detonation caused by low grade fuel, among other things.
Top end increases with less compression due to less negative work (not fighting compression at high RPM's).

 

[Husky390]

Bugger! said it the wrong way around, I meant "less squish". That's still surprising Ron. When you say "increased top end" do you mean more revs? Surely it is not more horsepower?

 

[jo360]

just bolt it up with no gasket and do the solder squish test and buy the gasket to suit how you want to run high/low comp.you want to be between 1.2 and 1.8mm each has its own plus and minuses, higher compression flame burns quicker better fuel economy run less advance but need better grade fuel gives more bottom end power but will not rev out as much.

 

[fran...k.]

Thanks Dave and Ron,
Very interesting.
I don't understand how top end increases with more squish. Is it more efficient with low grade pump fuel, revs out higher or some other reason?
I use 100 octane race leaded fuel for road racing.

The logic is pumping losses as compression goes up get to the point to where more power can be produced at higher rpm due to compression (being lower) ratio not the effect of squish in the way that modification is done. George/uptite has mentioned lowering compression to go over 100 mph on here at one time in talking what they did when these bikes were current. This logic may not hold true for all two stroke designs as much as for these.

Higher compression thermodynamically is more effecient. High octane burns slower but resists detonation better.

 

[Husky390]

Thanks everyone. I will go with lower compression as I always look for more revs.