I've recently machined my EXC 300 head in order sort out a small amount of damage on the squish band and also because I wasn't entirely happy with the first attempt that I made at machining it ages ago. I had written down that the chamber volume was 27cc and after my first go at machining it I kept it at 27cc but I closed up the squish gap to something like 1.2mm. 27cc actually gives a low CR of around 11.85:1 which is nowhere near the 12.8 or 13.2:1 that KTM quote, KTM telling porkies again! (The figure seems to vary depending on where you look). Anyway I re-worked the head a 5hit-load in order to get the squish to around 1mm and up the CR to around 13.2:1 (24cc chamber). In the end was a bit worried about the wall thickness of the finished head so I found a second hand one on ebay, it seems that this head is off a SX 300 kit, the chamber volume was 24cc as stock but the squish was still useless at around 2mm. I've machined this head to get the squish to 1mm, I've change the squish angle from around 2..5 degrees to 1.5 degrees, the volume ended up at 23.5cc which gives a CR of 13.5:1. Both my old head and new head made a big difference to the engine, no noticeable difference between the heads though even through one is a hemi and the other is trapezoidal in shape. I run 13:48 gearing so its a bit taller than what some people but the bike now wheelies off the throttle in 4th gear! (I know that you are not after 'mental' power!). Sorting out the squish whilst keeping the CR stock is actually a load more work than sorting out the squish and allowing the CR to go up.

If you do keep the CR stock but sort out the squish you will still get more power and the engine is will run better than standard. Pushing the CR up will obviously give you even more power. Some people reckon that it shortens the rev range a little but I've not noticed, I don't rev the nuts off my engine anyway as there is no point.

The downside of this and it has pissed me off is that I spent ages getting the crappy starter system to work, it was working well when the CR was around 11.8:1 but it won't turn the engine over now that the CR is over 13:1. I don't think that there is anything that will sort it out other than to lower the CR.

I would do your head for you but I have made a jig to hold my KTM heads and it won't work with any other head, I think that holding the head by screwing anything into the plug hole in order to turn it in a lathe is NOT good enough. (That's how I did my head first time around, it's just not accurate enough).

Below is the jig with one of my heads in it, I won't start cutting until there is less than 1 thou run out in any direction, ideally no run-out at all.

 

I would do your head for you but I have made a jig to hold my KTM heads and it won't work with any other head, I think that holding the head by screwing anything into the plug hole in order to turn it in a lathe is NOT good enough. (That's how I did my head first time around, it's just not accurate enough).

Many thanks for taking the time to answer so comprehensive. Much appreciated. And good to see you are still there.
The head of the Gasser looks pretty similar but I have no KTM head to compare. I will make a scan in the next few days and post the picture, you can then compare if you like.

Personally I prefer a correct squish band with minimal compression raise. The engine has enough power. Funny thing is that changing from roughly 2mm to roughly 1.8mm made a noticeable difference. Even the kicker was harder to push down (by hand). The engine runs better, responds crisper and smokes less. 1.8 is still too much but it seems 1.8 to 2.4 is the standard the factories deliver. One of the yanks measured 2.775 mm. No wonder that the engine was struggling....

@steveman - no tuning places on the mainland? Germany or Holland?

Miner my friend, not in Austria. Well there is one but I don't trust him and I think he is a massive c**t. Germany I have to check. Will see if I can get info on the German forum.

The good thing is, the bike runs so well with the other head that I am not in a hurry to further improve it. The idea was, that it wouldn't make sense to keep the head as a spare like it is. Plan B, get it machined well and install it whenever you like
Hope you are well and have no C19 problems in Abu Dhabishire.

 

Great info SCB... I’ve not checked the squish on mine but running the S3 head kit with the A insert head and it’s great. The AA head wouldn’t start off the button and cough stalled... they’re different chamber shapes/volumes vs stock though. Maybe give it a bash next time it’s in bits.

What year was your 300?

@steveman - no tuning places on the mainland? Germany or Holland?

 

Great info SCB... I’ve not checked the squish on mine but running the S3 head kit with the A insert head and it’s great. The AA head wouldn’t start off the button and cough stalled... they’re different chamber shapes/volumes vs stock though. Maybe give it a bash next time it’s in bits.

What year was your 300?

@steveman - no tuning places on the mainland? Germany or Holland?

Cheers for the compliment! My 300 is a 2013, I recall that some year models had pretty bad heads, it might have been the 2014 model. I have no idea what year the 300 SX head is but it seems like a decent design, it just had the usual issue of a massive squish gap.

 

Don't do this!.....

 

Holy....! This guy must be totally nuts.

 

Steveman, is the piston in your engine flat or domed?. The reason that I ask is because if you send your head off to be worked on then it is a bit harder to do the work without having access to the engine if the piston is domed. Closing up the squish whilst keeping the CR the same should not raise the PSI so it should not have been harder to kick over, I reckon that there must be a difference in the CR between the two heads, 'buretting' the heads is the only way to know this for sure.

What actual bike have you got?

You've made me 'think' about my head again, not having the starter working is pissing me off, I think that the head is coming off today to lower the CR a touch.

 

SCB,
the piston is very slightly domed. A friend called me and said he would like to try it. He works in an engine construction facility, has most modern equipment plus CAD/AutoCAD programs. He thinks it is not that complicated but I am not so sure. I would like to know how I translate the amount I lower the combustion chamber (for instance I lathe 4/10 off) into the amount of material I have to take off the combustion chamber.
My mate says, if he mills of a certain amount off the head, then taking the same amount off along the shape of the combustion chamber should keep the same compression ratio. And though that sounds plausible, I doubt that it is correct...

Currently I have a GasGas/19 EC 300 with a 2018 GasGas GP head. I assume that buretting means gauging the capicities and compare them?

 

All well here... strange times but could be worse... hope all is well back home and in the banana republic of Osterreich?

Things already easing here... [emoji41]

 

Hi Steveman,

If you want to close up the squish but keep the CR the same then it depends on how much of the combustion chamber forms part of the squishband, let say that the squish band in terms of area occupies 50% of the total area of the chamber then whatever you machine off the 'gasket face' you would then need to machine twice as much out of the chamber, this is because you do not want to machine anything off the squishband, if you did you could be back as square one!. Having said all that it is better to take a small amount out of the chamber and then use a burette and a plate of Perspex in order to actually find out what the volume of the head is. In order to make small changes and to clean up the machining marks you can use wet and dry and your fingers! (spinning the head in a lathe). Once you have the head setup in the lathe then do not remove it from the chuck until all the work is done, if you need to measure the volume remove the whole chuck from the lathe along with the head. Fit a spark plug in the head before you put the head in the chuck so that you can burette it. (I learnt this mistake the heard way!).

Personally I would work out what the CR is before you start, I wish I had done this rather than just believing KTM, that is why I kept my head at 27cc which gives a really low CR, I then ended up doing a load more machining which is why my first head maybe a bit marginal with regards to wall thickness. KTM are knob heads!

As your piston is domed I would take that into account, you could push the piston 10mm down the bore and seal it with some grease (measure the 10mm at the bore, not in the middle of the piston) then stick the Perspex plate on the cylinder with a bit more grease, then fill the space with water whilst measuring how much water is required. I think that your bore is 72mm to with the piston EXACTLY 10mm down the bore it should take 40.69cc of water. Lets say it takes 30.69cc, that must mean that the dome takes up 10cc. You need to factor this into your head volume. If the piston is down the bore at TDC then that needs taking into account too (My 'x dimension', deck height in normal language is as near as darn it 0.0mm). The reason that I'm saying all this is because your CR might be lower than what Gas Gas claim in which case you could push it up to say 13:1. With a working squish 13:1 is a safe figure in my opinion! Aim for a squish gap of around 1.2-1.0mm.

(I've just taken my head out to 24.5cc which is approx. 13:1, the starter motor can now manage to crank the engine for maybe 2 seconds before having a heart attack!

You can buy cheap burettes off ebay, you really should get one before starting any work on the heads, I had a glass one for about 15 years but bits kept dropping off it, the other day it finally broke beyond repair!. I've now purchased one of these....

RVFM Acrylic Burette 50ml | eBay

Type Stopcock burette. 50ml capacity. Capacity 50ml. Network Cables. Acrylic body is unaffected by dilute acids and bases except hydrofloric acid and ammonium hydroxide. The acrylic bodies and leakproof stopcocks of these dispensing burettes ensures high performance throughout their use.

www.ebay.co.uk

I've just googled the head for your bike, it appears to have the combustion chamber protruding from the gasket face, I guess that makes is a little bit more tricky to skim the head but its still 'do-able'. Having a flat face means that you can clean it up nicely with a sheet of wet and dry and a plate of glass, this would not be possible with your head. Buretting it would be a little more tricky to but again its do-able!

 

Sidecarbod, you are a star. Really appreciate that you took the time to explain all this to me. And I learned what a burette is. We call it bürette but I did not know what it was either LOL. For future head work consider using a syringe. You need that type they use in hospital for surgery as they are really very accurate. All you need is a mate working in a hospital.

What I did not take into account, is that the squish band is part of the combustion chamber, thanks for that heads up! That is a very important and essential point. If you allow I will copy and paste what you said/wrote and send it to my German mate.
I probably will not do anything to the head as the German GG dealer who owned a GG race team said the head is too thin to mill off more than 8 or 9 tenth. He said I should get in touch with VHM a company specialized on head works and they can even built a head for me with a dome I could choose. They offer either to choose the shape of the dome or build a head which you can machine your own dome.

As I had a fun project in mind, because I just have the head on the shelf all this gets to complicated and too expensive. It is strange that my Gasser runs better with the GP head which has exactly 1.78mm squish (vs 2.02mm of the standard head). Still too much, but it was noticeable. Maybe it is the shape of the dome and not the reduced squish. I could feel that the force I needed to push the kicker (by hand) was higher afterwards. As I changed the pilot jet at the same time maybe the bike runs better because of that LOL.

Concerning the original CR I was unable to find that out. No specifications from GG to get nor did anyone I asked (including dealers) know it. They were all guessing However, I don't need more power. The reason I started trying the GP head is that I had a too high fuel consumption and no matter what jets I used it id not get better.

I'll put that "project" on hold as it gets too complicated for just playing a bit. It would be easier to get an RK Tek head or a S3 head (250 quid where I live) instead of fiddling around...

Thanks again! If we proceed I will give you an update. Guess I owe you something...

edit: forgot to mention the combustion chamber does not protrude from the head, but will check that again when I am in the garage...