Custom thickness base gasket supplier?

[stevo135+]

Good evening all.

I haven't updated my Kawasaki KMX 125 engine build progress, mainly due to being so busy with work and home improvement and other projects. But basically I took the bike to a local two stroke Dyno tuner late last summer for fitting the Ignitec CDI and setting up the bike. The result was 23.6bhp at 8600rpm, and 14.52ib/ft at 8450rpm. A big improvement from the 18.5bhp I had before. The jetting was still a bit rich and I was running 1.2mm of squish after the cylinder head was re-machined to suit my 134cc cylinder.

Since then I've been working on a few details that needed further work. I've had the head skimmed so that I have closer to 1mm of assembled squish with the head gasket installed and head torqued up. My intention is to have a compression ratio around 14:1 and to try oxygenated race fuel like VP T2 as it's ethanol free and the oxygen content once jetted to suit might release a bit more power?

But first of all I'd like to try and get the deck of my cylinder skimmed slightly just to give the surface a better finish, and see if I can use a head gasket without sealant which previously was causing water leakage on the cylinder to head external join face.

My questions are:

1, what is the likely hood of being able to get my Nicasil cylinder skimmed just enough to give a nice clean deck face without it damaging the plating? Is there a special way this needs to be done?

2, Are there any suppliers of custom thickness base gaskets I can ask to make me some slightly thicker gaskets in case skimming the cylinder results in a squish clearance of less than 0.9mm? Ive spoken to a guy called Dan at "The gasket guru" but he has now stopped supplying gaskets due to other work commitments.

I'm not too worried about running a 0.9-1.0mm squish clearance and a touch more compression as the VP fuel is around 101 Ron+Mon average, and being oxygenated it should run nicely with more compression once jetted to suit and with the ignition map set up to suit it.

Any advice on my questions above would be really helpful and appreciated particularly on any firms I could approach to make some custom gaskets for me? Thanks Steve.

[JanBros]

higher compression always gives more power in 4-strokes.
in 2-strokes this is absolutly not true. it can be better, but it can also cost power.

There is finite energy in the burned fuel-mixture. With higher compression, more of that energy is used to push the piston down. that's a good thing. It also means that less energy goes into the expansion-chamber, and therefor the chamber suck's less and pushes less mixture back into the cylinder. less mixture in the cylinder is a smaller bang than the previous bang resulting in again less energy and so on ... at one point this will stabilyse but the end result is more power below the rpm-range of the chamber and less less power in the chamber-range.

On the other end : lower compression is less energy used to push the piston down, more energy going into the cylinder, more mixture being stuffed into the cylinder and a bigger bang next time. and so on ...

14:1 is pretty high for a mildly-tuned 2-stroke.

[stevo135+]

Thanks for your input and thoughts on the compression ratio for best performance within being reasonable about this being a mildly tuned road going engine.

With the 11.06cc of measured combustion chamber volume I had in the summer when I had the bike set up, my compression works out to about 13.1. It was more the fact that when I measured the squish it was still around 1.2mm as the 1.2mm solder pieces were barely touched by the piston. I thought that was too big a gap for the squish band to be doing any good at all?

By skimming the head I've now got an average squish clearance of 1.02-1.04mm using the 4pieces of solder method on the piston. I thought this lower squish will work better even if the compression does end up a bit higher?

I haven't measured the assembled clearance volume yet with this set up, but I seem to have a head volume reduction of about 0.4cc
This when worked out I think will put the compression ratio up to around 13.6-13.7cc. I also want to go away from having any ethanol in my fuel at all, so I thought the VP stuff which is pre-mixed with oil and also has about 6% oxygen content could help with both performance and safety? I know it'll need re-jetting though to suit the oxygen content.

I'm pretty happy with the bike as it is, but I thought just trying to get the last useful bit out of it without making it unreliable is worth my time especially if I am going to use decent fuel that two strokes seem to like? The Dyno tuner said that the ignition and fuel settings were very safe, and combined with the fact that my engine seems to only want to make peak power at ~8500rpm it should be very reliable. It's always in every configuration only wanted to peak at 8500rpm but I think that's another topic altogether?

Finally I've managed to get a degree wheel and measure my port timings as they differ slightly to the quoted figure. I use a 0.08mm feeler gauge to determine when the ports close, I'm not sure if that's the right way to do it?
Main exhaust port duration is: 180degrees
Auxiliary exhaust ports: 192degrees
Main transfer ports both are 126degrees.
I didn't measure the boost port but it's definitely lower than the main ports.

But my main concern at the moment is if it's possible to get my cylinder deck skimmed only enough to give a nice surface finish, or is it going to ruin my plating by risking it?
If I don't need to mess with it then great, but I'd like it to be a machined finish instead of as vapour blasted really. It has sealed ok with no water loss yet, but as I mentioned I have to use a light smear of sealant or I get weeping from the outside joint.

I think 25bhp might be attainable as a final result, but I definitely don't have the rpm to make much more power. That doesn't concern me though as the Ignitec CDI has stopped it dropping off like a stone, and it has a nice midrange for a 134cc engine.

[Paul250B]

Custom thickness base gaskets shouldn’t be a problem, there’s other suppliers who will do this for you.

With road bikes that are not optimised using different thickness base gaskets is quite often the way of getting the squish somewhere near. It’s not necessarily the right way to go about things though.

Optimised would see base gaskets used to correct port height or duration, port height because the fresh mixture is an aid to cooling the piston top, mismatched piston and ports can cause an eddiy, hampering the cooling effect of the fresh charge flowing over the piston top.
Biggest bang for bucks is always squish, the less mixture in the squish area the more mixture to burn = more power, cooling effect of head and piston (Bells book) if I remember correctly is best overlapped at .8mm to .9mm squish reducing the chances of detonation, with det being a possibility from 1mm and upwards. Recut chamber to correct compression ratio, excessive CR= pumping losses and increased chances of det.

[JanBros]

you can't determine correct squish with a fix number, it is dependend on the stroke.
the shorter the strok and the stiffer the crank, the tighter the squish can be.
idealy, the piston just doesn't touch the head, meaning there is no squish-volume at all.

squish = 1% of stroke minimum is a good guideline. and for a non-competition engine, make it a bit bigger.

[Paul250B]

you can't determine correct squish with a fix number, it is dependend on the stroke.
the shorter the strok and the stiffer the crank, the tighter the squish can be.
idealy, the piston just doesn't touch the head, meaning there is no squish-volume at all.

squish = 1% of stroke minimum is a good guideline. and for a non-competition engine, make it a bit bigger.

Yeah I made an edit and removed some information as it seemed to be getting too technical...

So I’ll add, do the math first, piston to port height should be calculated with expansion of rod in mind, as should squish, it’d be naive to set up an engine to run as cold.
With the math done 0.8mm is safe, over 1mm becomes unsafe due to det. Sure if you want to go with less squish then as I said biggest bang for bucks is always burning the mixture you have, correcting CR to save pumping losses.

[stevo135+]

Thanks folks for the information and help. I think that my target should be to get the squish clearance to as close to 0.9mm as possible then in that case. I'm not keen on having the combustion chamber altered again, it's already been re-machined to add 2cc to the dome. I've got another head as a spare that's been modified to lower compression even more with a 15cc dome.

I think using leaded high octane fuel with a higher oxygen content than pump fuel would allow me to run 0.9mm squish with whatever the compression ratio ends up at?

I'd have liked to get the cylinder deck face skimmed to give a nicer finish too, but I've got no idea if it would risk damaging the bore plating, though I'm told it won't by engineers I've spoken with.

The other thing is that I'm not really keen on lifting the cylinder with thicker base gaskets, as at BDC the bottom edge of the transfer ports and the main exhaust port both align flush with the edge of the piston. I've heard that it's poor practice to have the bottom of the transfer ports higher than the crown of the piston which makes sense to me?

Then again my Dyno tuner said that I've not got enough blowdown and or exhaust time/area to make power over 8500rpm. Raising the cylinder might help this, but then I suppose just raising the exhaust port roof would work just as well, and it wouldn't affect the transfer port heights and positions then. I've got 3mm difference in height between the main exhaust port opening and the auxiliary exhaust ports which are higher.

I'm no after a race bike, but with 23.6bhp now I think some optimization could get me to 25bhp as a reasonable target.

I need to hurry up and make some decisions as this is the third time I've had the engine apart for re-building or modification, and I've never even ridden the bike yet or had a full summer with it being back together in one piece.

I've joined the 2-stroke research and development group on FB, but it's all over my head and I'm very confused. I just want to get the engine back together and the bike ready to ride if I'm honest, and if I can make a dyno say 25bhp that would be a nice achievement.

[JanBros]

if you have enough blowdown time.area, why would you want to lift the exhaust even more ?
it would mean that something else besides blowdown is limitting more power at higher rev's. presumably transfer-time.area or length of the expansion chamber.

[stevo135+]

Hi Jan.

I don't know really and re-reading my post makes no sense as you say. I have I think about 27deg of blow down time according to my measured port heights. That sounds loads so I would assume that the cylinder can expel the burnt gases quite well by the time the transfer ports open?

I also read that 180degrees is theoretically the best exhaust port open duration, as it's 1/2 the stroke length which I heard gives the strongest expansion chamber pressure waves?

If I can get my 25bhp I don't care if it's at 7000rpm or 10'000rpm really. The only thing that I find strange is that this engine always seems to peak at 8500rpm.

It did that in near standard configuration, and now it's got a totally different set up. The airbox lid is drilled and cut to the biggest opening possible. It's also got a 30mm flat slide carb and a V-force reed valve now. The engine peaked at 8500rpm with a totally different expansion chamber and silencer too. And it's not the ignition causing it as I have a programmable system now.

The new pipe I was told was designed to suit the standard cylinder porting, and to peak at 9500 rpm, but it doesn't currently peak that high.

I take it that my high'ish compression ratio, could cause the peak power to come in at lower rpm, or the other option is that something is making my pipe run at a cooler temperature than it was designed to run at, but I don't know what would cause that?

[JanBros]

those 27° are only a part of the timing.area
as the name says : it's about time (the higher the rev's, the less time) and area (basicly hight x width)

did you change your exhaust timings ? as higher timings need a longer pipe. meaning your pipe is too short now and than it's normal that peak power comes in even earlier the the rpm the pipe was disgned for.

[Professor]

I had my KR1S barrel top faces lapped to be a precise surface and finish as they were crudely finished slightly convex from being replated and were causing head gasket leaks.

I'm not sure how common lapping machines are but the barrel may well need a jig making to hold it square to the lapping machine so the face is perpendicular to barrel and parallel to base gasket. The face was a perfectly smooth finish with out any problems on the nikasil plating and I've not had any gasket problems since, skimming the base gasket face of the barrels prevented my base gasket creep problems