Compression

Dirt Bike Technical discussion. This is for a little more in depth discussions :-)
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60something
Posts: 54
Joined: Sat Mar 03, 2018 6:45 pm
Location: Midwest USA

Compression

Post by 60something » Sat Mar 17, 2018 9:17 pm

In my 1987 K5 rebuild, the cylinder was in decent condition with the stock bore. I had Millenium replate & size it to a new '88-up std size piston, per the info in Sandblaster's excellent post regarding model year vs piston characteristics. I was wondering , as I am not planning to have the head milled to restore the compression ratio with the new piston, would the resulting lower compression ratio be substantial enough to make it easier to kick start? I have pretty much resigned myself to getting the head mod done for the compression release, but just wondering... :?:
I don't know half of you half as well as I should like, and I like less than half of you half as well as you deserve.

2006 KDX200 qty-2
1987 KX500
1991 CB750 Nighthawk
1973 H2 750 Triple - gone but not forgotten

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Sandblaster
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Re: Compression

Post by Sandblaster » Sat Mar 17, 2018 11:18 pm

Your likely going to see reduced compression over the dome style piston.
Will it be easier to to kick over?
If the engine was totally stock then likely yes.
If it's been modified it could be anywhere.
Remember, for every .010" of clearance from the piston to the squish band you gain a volume of about 1.5cc..
When you get it put together and broke in, put a compression gauge on it and tell me what the PSI is.
If the uncorrected compression ratio is 13.5:1 then the cylinder pressure should be approx 198lb.
Most of ours come in just a hair over 200 for a relatively stock engine.
Kawasaki says the usable range is 105psi-164psi so anything over 164 should be awesome 8-)
If bikes are for kids I'll never grow up.

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60something
Posts: 54
Joined: Sat Mar 03, 2018 6:45 pm
Location: Midwest USA

Re: Compression

Post by 60something » Sat Mar 17, 2018 11:30 pm

Cool. Thanks!
I don't know half of you half as well as I should like, and I like less than half of you half as well as you deserve.

2006 KDX200 qty-2
1987 KX500
1991 CB750 Nighthawk
1973 H2 750 Triple - gone but not forgotten

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