how much can i safley skim of the head 73 gt750

[two-stroke-brit]

so my crank seals went and i know have it apart again,
just thinking of doing a few mods before rebiuld .
i think the later models had increased compression ,?
was this done by skimming the head or reducing the cylinder hieght.
either way how much is safe to skim off?.
thanks in advance
mark

[tz375]

3mm according to Suzuki, but I have difficulty with that number. I took about 0.75mm off the block to set deck height and used a 0.5mm thinner head gasket and then took around 0.75mm off the head for a total of about 2mm and that gives me a cranking compression of around 165psi which rather a lot for that squishless head design and is around 12:1 static.

That's a touch on the high side for sustainability. I would aim to take off a total of say 1.0 -1.5mm overall.

[oldjapanesebikes]

3mm ? Not sure about that Richard - service bulletin GT-7 talks about 0.2mm being removed from the head and then using a thinner head gasket giving a 0.9mm total reduction.

[tz375]

Ian, Good answer. That's the changes from L to M model. the 3mm is mentioned in one of the articles on line. It may have been in the Performance section of the Clymer manual.

[oldjapanesebikes]

Ian, Good answer. That's the changes from L to M model. the 3mm is mentioned in one of the articles on line. It may have been in the Performance section of the Clymer manual.

Found it - page 152 of the fourth edition of the Clymer book - 1mm is recommended, but it mentions 3mm as being possible. Not sure I'd want to try that on a street bike

[Barry S.]

so my crank seals went and i know have it apart again,
just thinking of doing a few mods before rebiuld .
i think the later models had increased compression ,?
was this done by skimming the head or reducing the cylinder hieght.
either way how much is safe to skim off?.
thanks in advance
mark

You could just get the thinner head gasket from the later models, that would help. Heads sometimes goes cheap on Ebay, you could replace the head as well.

[two-stroke-brit]

thanks barry,richard,ian.
my plan is to take about .5 mm of cylinder and get a thinner head gasket and see what compression that gives me.
aiming for about 140/150 psi.
if i need more i can always do the head as well.
what is the standard head gasket thickness for the later models?
as i will probably go to cometics for a copper gasket
T I A Mark

[tz375]

That's a good question. Late gaskets were 0.8mm, Cometic are 1.09mm and Athena are 1.3mm. Copper are typically .040" or 1.0mm.

I would order a Cometic gasket in the material, thickness and bore that you want and work form there. Stock and most gasket set gaskets are way too large in the bore and that encourages detonation.

[Suzukidave]

I would say skim the head as if things dont work out its a lot cheaper to replace the head than the block .

[tz375]

With a GT750, it's better to skim 0.5mm off the block to get zero deck height and adjust the squish...... Uh oh. There is no squish, so it doesn't matter too much where it comes off, but don't use a gasket set head gasket if you raise compression. They are typically too large in the bore.

Get a Cometic gasket instead and specify the thickness you want.

[two-stroke-brit]

thanks guys i have to take the block to work anyway ,as i need to blank the sris nipples and repair a couple of stripped threads with inserts.
what do you think about changing the center seals for ali lab seals ?.
cheers mark

[oldjapanesebikes]

what do you think about changing the center seals for ali lab seals ?

Mark - this relates back to your early seal failure problem ? I still really think it has more to do with a crank re-build or possibly an engine reassembly issue rather than a fault with the seals. Take a look at this thread as I suspect he has nailed the key issue - if the web shoulders have not been properly cleaned up then new seals will just wear out very quickly. Normal seal life is measured in the 30,000 to 50,000 mile range with some owners getting well over that. Just my own two pence worth.

[two-stroke-brit]

thanks ian. the crank was rebuilt by bill bunn so i think they would have addresed any corrosion issues when installing the seals.
and i dont see where to look in those pics for the web he is talking about .( could you mybee popst a pic with an arrow for the hard of knowong where to look.)
i cant see any corrosion on the dissmantled crank,
and my crank was in good condition rust wise before i sent it.(admitedly i couldnt see under the seals)
thanks mark

[tz375]

Lab seals work between cylinders but that only applies between 1 and 2 where there isn't much room . 2 to 3 has the primary gear, so no lab seals there.

Your seals that failed, were they NOS or new? I read somewhere that the seal material changed to better deal with modern fuel, but I have no way to know if that really happened or is just an internet myth.

If the crank is going back to Bill Bune, ask them to examine the areas of the crank that the seals sit on and to return your old seals so you can see what's going on. Are you sure the seals are shot - did you do a leak down test and can you hear the air escaping between cylinders?

[two-stroke-brit]

thanks teazer the seals were new from sudco and i did the leak down and one cyl held for about 4 mins but the left and center wouldnt hold at all.
on a related note if i change the the 8mm stud that sticks out of the block to a bolt like the other 8mm one in the head
would that cause an issue ,i dont see how it would but they must of done it for a reason??.
thanks mark.