Hello! all! i was reading in my old clymer manual with the "performance how to" chapter, and it said you can mill 1mm off of the head to raise the compression ratio. has anybody done this? my head is off of a 1977 gt 750 B which if i remember has higher compression than previous years? if i milled 1mm off would reliability become an issue? My buffalo is .50 over and has a 3 into 1. in the book they say up to 3mm can be taken off but that seems like alot.
any advice would be great! thank you!
GT750 Head milling
Moderators: oldjapanesebikes, H2RICK, diamondj, Suzsmokeyallan
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jaycee69
- Around the block
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GT750 Head milling
73 GT550 K
77 GT750 B
88 Yamaha YSR50 (Street Legal)
91 Yamaha YSR80 (Street Legal)
77 GT750 B
88 Yamaha YSR50 (Street Legal)
91 Yamaha YSR80 (Street Legal)
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Coyote
- Moto GP
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- Suzuki 2-Strokes: GT550x2, GT750, GS1000
- Location: Tulsa, Oklahoma
Re: GT750 Head milling
I'm so far from an expert its ridiculous - BUT- I think you would get a larger gain with one of those riser blocks between the crankcase and cylinder block. Changes the port timing.
I don't think added compression helps these old 2 strokes much, but you don't have to worry about bending a valve.
I don't think added compression helps these old 2 strokes much, but you don't have to worry about bending a valve.
I was born with nothing and still have most of it left.
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1978 GS1000C
1976 GT550 ongoing money pit.
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1978 GS1000C
1976 GT550 ongoing money pit.
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jaycee69
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Re: GT750 Head milling
Do you know of anyone who has done either? would your compression go down with a riser block? or would it stay the same? how would it affect the power?
73 GT550 K
77 GT750 B
88 Yamaha YSR50 (Street Legal)
91 Yamaha YSR80 (Street Legal)
77 GT750 B
88 Yamaha YSR50 (Street Legal)
91 Yamaha YSR80 (Street Legal)
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rngdng
- AMA Superbike
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- Location: Blythewood, SC, USA
Re: GT750 Head milling
When you use a lift plate, you have to mill the top of the cylinder the same amount. I have a lift plate in my 750, a milled head, and 1.5mm off the rear skirt of the pistons. Also 34mm slide-valve carbs; all on a 75 model engine. It dyno'd at 80HP. That said, yours will never get that high with a 3 into 1. I suggest you enjoy it as is. It's a lot of work, a lot of money, and it's really not worth it. If you want to go fast, get a modern bike.
Lane
Lane
If you stroke it more than twice; you're playing with it.
Too many bikes, too much time, ENOUGH SPACE, FINALLY! Never enough money.........
Too many bikes, too much time, ENOUGH SPACE, FINALLY! Never enough money.........
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GT750Battleship
- Road race school
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- Suzuki 2-Strokes: Suzuki GT750A 1976
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Re: GT750 Head milling
+ 1 with Lane, enjoy the GT750 for what it is,a very good '70's Supebike,but don't tell everyone...it only pushes prices up not a bad thing if you already have one 
Cheers,
Roger
GT750Battleship.
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tz375
- Moto GP
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- Location: Illinois
Re: GT750 Head milling
A lift plate will raise all the ports for more top end power, but with a 3 into 1 you won't get there.
Raising ports lowers the compression when the motor is not on the pipe and that loses low and mid range.
Raising compression helps all through the power band and is proportionately more at lower to mid revs. My motor is ported and has 0.5mm off the block and a fair amount off the head and even with a 3 into 1 it works well and with spannies it works even better. Beware of stock head gaskets which are too large in teh bore and you may need to retard the ignition timing a touch, so watch out for detonation with crappy fuel.
Raising ports lowers the compression when the motor is not on the pipe and that loses low and mid range.
Raising compression helps all through the power band and is proportionately more at lower to mid revs. My motor is ported and has 0.5mm off the block and a fair amount off the head and even with a 3 into 1 it works well and with spannies it works even better. Beware of stock head gaskets which are too large in teh bore and you may need to retard the ignition timing a touch, so watch out for detonation with crappy fuel.
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jaycee69
- Around the block
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- Joined: Sun Jan 15, 2012 11:33 am
Re: GT750 Head milling
im not going for more speed, just more punch, torque. overall more low end grunt.
sorry for renewing this thread,i got back to working on my buffalo......Four mounths later haha
sorry for renewing this thread,i got back to working on my buffalo......Four mounths later haha
73 GT550 K
77 GT750 B
88 Yamaha YSR50 (Street Legal)
91 Yamaha YSR80 (Street Legal)
77 GT750 B
88 Yamaha YSR50 (Street Legal)
91 Yamaha YSR80 (Street Legal)
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rngdng
- AMA Superbike
- Posts: 1769
- Joined: Tue Oct 14, 2008 6:47 pm
- Location: Blythewood, SC, USA
Re: GT750 Head milling
Your setup is about as torquey as it will get. You can get more punch off the low end with a smaller counter-shaft sprocket. Maybe that will give you what you want.
Lane
Lane
If you stroke it more than twice; you're playing with it.
Too many bikes, too much time, ENOUGH SPACE, FINALLY! Never enough money.........
Too many bikes, too much time, ENOUGH SPACE, FINALLY! Never enough money.........
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tz375
- Moto GP
- Posts: 6212
- Joined: Mon Nov 03, 2008 10:47 am
- Location: Illinois
Re: GT750 Head milling
Just as a point of reference, I have built only 4 street motors so far and on all of them I raised compression by milling the block and head. You may want to order head gaskets from Cometic and fit an electronic ignition, and you will find that raising CR helps a lot at low to mid revs.
