S3 cylinder head

[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