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Best way to lower the CR?

kochic

kochic

Forum Mentor
Hi,
What is the best way to lower the CR on a 13.5 to 1 kit?
Sent them to the manufacturer to have the top of the piston machined?
Or to have a thick aluminium base gasket?
Is this safe to remove material from the top of the piston?
I have oversize in and ex valves (40/33) so i have to modify every piston i choose.
My bike is a gs1000 2V.
thank you,
Marc
 
You could also stack 4 or 5 base gaskets to lower the comp ratio to a usable street ratio. Ray.
 
tkent02 said:
Sell it, get what you want.
Someone will want those pistons.
Click to expand...


This is what the other guy doing.I was looking for info if it 's worth buying or not.

To Ray, is it better to stack copper base gaskets or have a thicker one made?
I know you like to use the OEM gaskets, do you use them for this?
Thank you,
Marc
 
Get a custom set of CP pistons for your desired CR and valve pocket size. =)
 
#1 thicker head gasket
#2 thicker base gasket
#3 cut the piston crown


have an aluminum or copper base gasket made. I've seen a .120" base spacer on a KZ with XS1100 pistons in it. to get good deck clearance and squish.

you can stack metal base gskts. but it may seep a little.

The thing about spacing is you are going to lose some power as the squish band effectiveness is diminished with the excessive gap around the circumfrence of the combustion chamber edge.

getting a lower comp piston or cutting the ones you have is going to retain the most power.
 
How far do you want to drop the compression ?

If just alittle you can retard the intake cam causing a later valve closing resulting in lower compression. That's what I do on my Katana Dot head with 13-1 pistons, I can advance it to around .040 V/P clearance and get 265 psi or retard the intake cam to around 200 psi resulting in more V/P clearance. I don't know the V/P limits on a 2 valve motor so maybe my 16V head is a bad comparison ??

It's a fine line, go to far with retard and she'll run like a dog.
 
Last edited:
Lower CR

Lower CR

Can't you just beat the dome down a little? Steel hammer it? Okay, just kidding. Check the thickness under the dome and valve pockets. If over .150" thick, it can be modified. Calculate the CR needed after measuring chamber volume and actual dome volume. Perhaps, only a slight reshaping of the dome is needed to get a streetable compression.
Laters
G
 
Depends on the brand of piston. Wisecos are built on a pent roof forging and you can not cut too much off.
 
Hi Jay,
Do you still sell the 1176 mtc kit?
i dont know the thickness of the 1175 wiseco piston head-dome but the 1100 wiseco i already have are thin is that area.I dont think i can remove a lot of material except for the O.S. valve pocket.
Marc
 
Pist CR

Pist CR

After making and designing pistons for 20 year, I have seen many different designs. Since no fotos or measurements were supplied, we are just guessing randomly. They may be nice, thinwall pentroofs or could be solid tops. Maybe, sell them and get what you really need. Spacing the head up an 1/8 of an inch while workable is not really the best way. Maybe actually cc the head and cc the piston and see what CR you really have. Then decide if it can be cut down or more cost effective to actually get what you really need.
Laters
G
 
Hi G,
i dont have the set in hand.I have 1100 wiseco piston set and a member here sell a new set of 1175 wiseco piston with sleeve for cheap, at least here in Canada.But the CR is 13 to 1 and i want to ride the bike on street.
I think its better to buy want i really need rather that spend money on piston that i cannot modify at home.
Thanks,
marc
 
Marc, I have a set of used MTC 1175cc pistons. They are the 10.5/1 comp ratio...Very useable but may need rings... I ran them on the street....My garage is a gold mine for GS1000 race parts....collecting dust....PM me if you want.....Billy
 
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