Hello there folks,

My 99' GS500 has low compression. About 95~100 PSI per cylinder. It looks like I'm going to need a rebuild, while I'm at it I was thinking of milling the head to get a higher compression ratio. I'm wonder what the maximum you can mill the head is?

After what point do you have to start cutting deeper valve/spark plug recesses?

After reading a few posts on this subject it seems most people shave about 0.8mm, which from my calculations gives you a CR of approx. 10.3:1. I was looking for something closer to 12:1. Shaving 1.5mm off the head will give me an approx. CR of 11.8:1.

1.5mm seems like alot. Any help is appreciated.

think i would concentrate on make it breath better rather then bumping CR.

Bore,port,and cams would give you more and still be reliable on pump fuel

whare are you at, as I have a good motor that you could swap in there.

Not to make an obvious comment but make sure that your testing the compression when the bike is warm.   A cold bike will give you readings around that pressure reading.

Also milling the head 1.5mm is quite alot and you will definately have to adjust the timing on the cams so you dont pinch a valve off.

1.5 mm seems a huge amount to me. The old trick is to put plastercine or modeling clay on the piston crown and turn the engine over a few times, the valves will make an impression in the clay and you can then judge how much clearence you have. For very accurate readings you might be
able to use this stuff if you can get hold of it.

http://www.plastigauge.co.uk/

Even with high comp piston, .417" lift pistons, milled cylinder and a milled head, there still was over 1/16" valve/piston clearance. I never measured the head though. I have been moving stuff around in my shop and when I come across it I will see if I can get a good reading. It is not on a motor since it has a bad valve seat.

just put some 1027 gsxr pistons in it and mod a oil cooled GSXR750 head to fit on it.

Quote from: falcon0321Not to make an obvious comment but make sure that your testing the compression when the bike is warm.   A cold bike will give you readings around that pressure reading.

Also milling the head 1.5mm is quite alot and you will definately have to adjust the timing on the cams so you dont pinch a valve off.

something even less obvious is having the throttle held WFO.. (WOT)..  it made a HUGE difference on my gs. I don't remember what the before/after was, but it was enough to REALLY freak me out at first...

QuoteBore,port,and cams would give you more and still be reliable on pump fuel

For the hotter cams you need a CR of at least 11:1, if I recall correctly.

Quotewhare are you at, as I have a good motor that you could swap in there.

Barrie, Ontario.

QuoteNot to make an obvious comment but make sure that your testing the compression when the bike is warm. A cold bike will give you readings around that pressure reading.

It does get slightly better when warm, up around 125PSI. My manual says the service limit is 114PSI, standard is 142-199PSI. It's winter so if I'm going to need a rebuild I'd rather do it now.

Quote...snip...Even with high comp piston...snip...

Are these the Kawi GPZ pistons I've read so much about?

Quotejust put some 1027 gsxr pistons in it and mod a oil cooled GSXR750 head to fit on it.

Will do.

Quote...snip...having the throttle held WFO...snip...

Throttle was wide open.


Thanks for all the comments. I'm asking because I can get machine work done for free through family connections. Adjusting the cams wouldn't be a problem as I'm good with math and have the technology.

So nobody knows the valve-to-piston clearance on a stock engine?

Thanks,
Chris

Quote from: Chris_B

Quote...snip...Even with high comp piston...snip...

Are these the Kawi GPZ pistons I've read so much about?

They were 79mm V&H/Wiseco pisotns, the ones not made anymore. The GPZ pistons are 78mm and have basically the same profile.

Comparison pics:
Top View
Angled View
Front View
Side View

QuoteFor the hotter cams you need a CR of at least 11:1, if I recall correctly.

cams dont care what the CR is, its just timing the opening and closeing of the valves and how high they are open.

if the valves open wider and longer, more air will flow in for the most part. besides, buy going over to GSX head and pistons, you can use the GSX cam profiles