I have a 900SS carb model engine rebuild going on that's taking a bit of my time. I thought I'd be clever and pull the base gaskets like a lot of people have said they've done in the past. The aim of this rebuild was an engine that needed no more octane than the base unleaded offers, which was why we fitted new rings to the original pistons as opposed to fitting the JE high comps that cost about the same as a set of rings from Italspares (much cheaper from Ian than Ducati). But part of the squish setting theory is that a tighter squish setting makes an engine less likely to detonate, even though by definition it increases the compression ratio as well. That's what I keep telling people anyway, and I have to (should?) believe it myself.
So I checked the squish using clearance wax and it was a touch over 0.8mm at the very outer edge of the piston with no base gaskets. Which is right on the limit for as tight as you want to go, but as explained below, it's much less of an issue in a 2V motor than the 4V.
The 2V motors have a tapered squish area and a somewhat corresponding taper on the piston crown, but it's not the same angle. Torbjorn has been playing with an ST2 recently and I think he said the head taper was 7 degrees from horizontal and the piston 3 degrees. Something like that anyway. So the closest point is the very outer edge of the piston, and a couple of mm in from the edge is over 1.0mm.
The change in compression due to removing the base gaskets (originally 0.3 and 0.4mm for the horizontal and vertical respectively from memory, no idea why they're different) is 0.4 for 0.4mm variation. I checked a std 900SS a long time ago and I think it was around 9.3:1. With the gaskets out it'd be 9.6:1 on the horizontal and 9.7:1 on the vertical. Not a big increase, but reportedly noticeable on both the road and the dyno. Compression really is the thing that brings these 2V motors to life.
In my experience, a change in compression such as you get from adjusting the squish 0.3 - 0.4mm gives a more noticeable change on the dyno at higher rpm. This is useful if you're chasing a number, because you might see 2, 3 or 4 more. Which helps when you have an owner sweating on it (or expecting more than you know you're going to get). The trade off comes in a similar reduction in piston to valve clearance, reducing the amount of inlet cam advance you can run.
I was happy with 0.8mm squish clearance and assembled it with the base gaskets out. I used the normal threebond 1215 to seal the cylinders to the cases and jammed the heads on. All simple enough. I went back later that night to check and set the cam timing and, typically, it all went pear shaped. I checked the horizontal first off, which was 114.5 degree inlet centreline and 1.5 degrees retarded from spec. Not unexpected, as dropping the cylinders moves the cam closer to the timing shaft which, when tension is applied to the belt, rolls the cam back against the direction of rotation, retarding it.
Then I popped the belt off and checked the piston to valve clearance, seeing how far I could wind the valve open at given piston points after TDC. Generally, in my experience engines are tightest in the 5 to 15 degrees ATDC range, so I check from TDC to 30 or so degrees after on the inlet. The exhaust is irrelevant, as advancing the cam moves the exhaust valve away from the piston.
Which is when it went bad. Even at 114.5 it was at the tightest I would set them clearance wise. Leading to a short session of wtf? followed by some swearing and general grumpiness. Given it's a 2V motor with no head gasket the obvious solution was to pull the head to see if I'd put the piston in the wrong way. I was pretty sure I hadn't, but the first thing to point the finger of blame at is me. The exhaust valve cut out is usually smaller than the inlet due to the smaller valve and sometimes shallower (not on these pistons, as it turns out) and if you fit the pistons 180 degrees out you get the exhaust where the inlet should be.
But that wasn't it. Comforting to know I'd done it right, but disappointing that I had no answer. I went to check the parts lists to see if the small valve 900M motor had different pistons in case this motor had them, but the number was the same 93 onwards. So I was somewhat out of ideas. It's just a simple mechanical device, so what it is is what it is.
The issue is that, in my experience, on a road bike, the effect of advancing the inlet cam is more noticeable than more compression. Especially in the rpm range this bike is going to be used in. Leading to the obvious conclusion of needing more piston to valve clearance and there's only one way to get that.
I did find some 78610061A base gaskets in my stash which are labeled as 1992 900SS and measured 0.4mm thick. The listed horizontal (78610391A, 0.3mm) and vertical (78610471A, 0.4mm) base gaskets are the touch different, mainly the small cut out in the inlet side of the vertical to allow for the second oil return from the head the vertical has. The 61A has the same shape as the 391A, so I fitted one of them to the horizontal to give 1.2mm squish at the outer edge on both.
I removed the cylinder, cleaned all the goop off, gooped up the gasket and refitted the cylinder and head. I was surprised how much force was required to seperate the threebond holding the cylinders to the cases. I couldn't pull them off, and had to go the tyre lever.
Checking the clearance and timing again I now had an extra 0.4mm clearance and the cam was now at 111.5 degree inlet centreline. I fitted a 2 degree offset key to get 109.5 degrees, which is about 4 degrees off the 106 I would expect one of these motors to allow. Well, I would have expected a little more as the valve seats and faces had been cut, dropping them about 0.2mm from original based on the changes made to the shimming. But, by this point, I was well over thinking about what was wrong (or should I say different) with it and still had the vertical to do.
The vertical cam had originally been at 119 degrees inlet centreline with no base gasket. With the 0.4mm base gasket fitted, it was 117. An 8 degree key gave 110 and the conclusion of the episode. A nice way to spend a few hours that I can't charge the customer for. Experience can be expensive to come by at times. I'll be curious to try it again, but I'll do the full test assembly first before getting the goop out.