S3 cylinder head

[motopsycho87]

Hi all, can anyone tell me if the squish on the S3 head inserts is different between all of them of the same? I'm looking to buy the S3 head as I like the idea of changing compression ratio to alter the power band (I use my bike as SM and for enduro), I know I'm going to have to machine it for correct squish, but is the only difference in the chambers or squish height as well? Cheers

[dANbOT]

I have an S3 head. The higher compression inserts certainly have less squish clearance, I got my squish down to about 1.6 with a thin base gasket and the AA insert (highest comp) Runs great @ 3000 - 7000 feet on 91, any lower and you will surely need better fuel.
I believe the coolant flow is improved as well, I rarely boil over any more.

[Moto7man]

Jakobi has an extensive thread with photos on the squish for the S3 heads. More comprehensive than anything else on the web.

[motopsycho87]

I've had a hunt through the post but can't find a list of squish clearances or anything... Am I being blind?

[dANbOT]

For what it's worth, here's what I was told as I ordered my S3 kit:
-The shape, and volume of all 3 inserts combustion chambers are identical, except for the depth they reach down into the cylinder (affecting both squish, and compression ratio.
I can't confirm this, but I CAN tell you that the squish is reduced with each step to higher compression, and otherwise they look the same.

If your a perfectionist, and want to achieve 1mm of squish, you would need to have some work done to balance squish, and comp ratio on the S3. Additionally, if you cannot use the higher comp inserts due to elevation your probably going to be close to OEM squish measurement. (mine was over 2mm in stock form). At this point, you may as well just send out your OEM head for machining, to achieve what your looking for.

If however 1.5mm or so is acceptable to you, the S3 will work great. I have zero complaints, and my 300 runs noticeably stronger than stock with the "AA" insert at 3000 feet +. I also feel it runs cooler, but there's too many variables to be sure.
Again, with the "AA" insert, a thin base gasket, and a Wossner piston, squish on my 2014 300 ended up at 1.6mm

[motopsycho87]

Sounds like there is no point at all then, I was hoping the squish would be the same on all and just different dome volume, but there's no point if I'm going to have to machine each insert after machining the head. Nice thought anyway!

[memphis2857]

I think Jacobi did something similar with his S3. The problem you will run into is machining the head itself AND the inserts because each price accounts for a portion of the mating surface.


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[Jakobi]

Correct.

The actual mating surface of the head against the cylinder includes the insert as well. So if you were to machine down the outer to reduce clearance, you'd also need to turn the outer edge of each insert down to match.. so you're machining 4 x parts so that they all match each other order for the head to simply seal against the cylinder O-rings.

Now that you've closed the overall clearance by machining the head mating surfaces down, you still have 3 inserts with varying squish clearances (by design). So are you going to machine 2 x inserts down to a standard clearance?

You then have 3 x inserts that will have UCCR well above target.. so you'd have further machine work adjusting the volume to have 3 x variable head inserts while maintaining the same given squish clearance, with differing compression ratios.

With a 0.5mm base gasket, and the highest comp insert, it closed my squish clearance down to 1.25mm. From here I simply had the volume corrected to reduce the compression back down (I think around 13.5:1 from memory).

It's a lot of work involved to try and get the setup you want.. for someone with the tools, time, and knowledge it could be a fun task. If outsourcing and paying someone else to do the work for you it's not worth the outlay.

[motopsycho87]

I'm the machine shop manager at a local foundry so tools and time are not really a concern. Main thing I'm worried about is it looks like I'm going to have to take 2mm off of the head. I've done it with the standard head, but I also spent a lot of time playing around with it trying to get the RM125 CDI to work really well (don't bother!), so there's not a lot more I can skim off.

Motocross Centre have the original heads for about ?80, but 'not in stock' sadly, so looks like my only option is the S3.

Currently I'm running 28cc (approx, only measuring in 1ml increments), which is about 11.45:1. Ideally I'd like to make a 12,13,14:1 set.

[motopsycho87]

Still waiting on the damn thing to turn up, trialendurodirect are normally really quick