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I have my engine running good, its a fishing motor and is running as good as they come.
its got the large carbs and runs very smooth and picks up straight away, so pretty good.
hole shot is fantastic as too is the mid range, but I was hoping for a bit more top end rpm.

now I want more power if possible and I thinking upping the compression might be the only way to go for
just a bit more top end power without stripping the whole lot down.

Mine is the later 56 cu in oval port 3 cylinder with the one piece head (looks like 3 but is just 1).
There is no option for changing engines or messing around with port work ( apart from a slight angle on the tops)
The seam inside the block behind the sleeves will be gone, but will be keeping the exhaust reliefs for the idle so as
the engine can still be used for slow trolling when needed.

I have a spare cylinder head to play around with that needs skimming big time, the engine it came off was blown on the
middle cylinder and this head has some severe pitting on the middle squish area.
There is no pits at all in the combustion chamber itself, but this head would need around 1.5mm taken off.
so that's around 60 thou +/- a bit.
if the compression would be too high to run pump gas then maybe I can mill out the combustion chambers a
touch to compensate and bring it back down a touch.
the very deepest pits are 1.5mm

what I am after is as much compression as possible but has to run on pump gas !!!
AV100 is far too expensive here and hard to get hold of.

The compression and im getting at the moment is 141 -143-142, I am wondering what would I get with 60 thou
shaved off a head. ?

I know the sst60 has a thinner head than the fishing motor, but that also has a shaved deck/block to give its
higher compression.

I have been told before that the heads can have as much as 80 thou taken off without any problems regards the
water jacket. but I dont know if this could run on pump gas and what compression would be

so my questions realy are
1 /what amount of compression difference would I expect to get with 60 thou taken off
2/ would this then still run on pump gas. ?
3/ what sort of power gain would the higher compression give. ?
4/ how much thinner is a sst60 cylinder head to the normal fishing motor head.?
5/ can the combustion chamber be milled ? and by how much ?
6/ is there an option for thicker cylinder head gaskets available ?

I know its a lot of questions, but I am curious to is this worth trying to get more power, or atleast give me a
spare cylinder head for in future if needed.

getting spare heads over here is not exactly easy for the 56 cu in.
at the moment this is scrap unless skimmed by a good amount. 60 to 70 thou !!!

here is a couple of pics of the offending head, a friend of mine is willing to mill it when he gets his new mill.

WARNING THIS IS A SAD LOOKING HEAD THAT HAD BEEN ABUSED ( need to make it good again )

First, raising the compression will not give you more top end speed. You will gain low & mid range torque for better acceleration, but not gain on the big end. You can mill as much as .0125" or 3+ mm and not have water jacket issues. The SST-60 has a pocket depth of 0.450" +/-.010"
The stock heads are in the 0.560" range. Of course you can not get away with more than .060" to.080" with stock ports and still keep compression below 160-165 psi. You can use 93 octane pump gas with 150-160 psi with no problems. I would suggest an additive such as ACES IV for 2 strokes. You add 1 oz to 6 gallons of fuel. It is pricey, but you only need a little to fill your tank.

We run pump gas in our T-850 boat Team Hydromotive (Todd) with no issues and our compression is 150-160 psi. Also, our 2 cylinder T-750 runs pump gas with 150-160 psi.

If you want to sell your head, we'll give you 50.00 USD. We'll be in Waterford this weekend for the boat races. You can bring head and watch the races, either Sat. or Sun.

Good luck,

Thom

Almost forgot to mention there are aftermarket gaskets that are .015" thicker than the stock OEM gaskets. Sierra Supply is one source to check out.

Great info hec2buck
cant get to Waterford as I live near London England 3500 miles away on other side of atlantic.
would love to come there one day and watch some fast stuff on the water.

as for the head I will let my friend have a go at milling it down when he gets the mill and keep it as a spare.
good to know about the thicker gasket too.
ive got good punch from the off and mid range is brilliant, maybe have to settle for what ive got.
its not a race hull its similar to a whaler 13 classic with more v in the centre that dont like to come up.

thanks again for the info

Glad to help.

Thom

might sound daft but when you talk of the octane rating are you talking RON , MON, OR aki (R+M)/2 ?