FBF Technical question

Ducal

Hi guys,

Just in the process of finishing rebuilding an FBF built 916R 955 motor. The pistons with .2 cylinder base gaskets were hitting the heads. Fast rewind and a check in the never to be used again department......... Sure enough the old base gaskets were still there, I probably kept them waiting for scrap copper to go through the roof..... but collectively they measure 1.6mm which amazingly is the thickness of what we need to make it all work.

The question is bearing in mind everything looks as it should, period, is there any reason why this would occur. I will add at present I have not measured the rods, but they are Pankl for sure. I am only a layman, but if there is a performance advantage I would be interested to know. I hear also that back in the day the geographics sometimes dictated that fixes were subject to part availability. Thanks in anticipation

griff851

The squish clearance, distance from valves in a full cycle, is dictated by the barrel base gaskets.
Stick on your old barrel base gaskets, lay solder(1.5mm thick) across the piston front to back and left to right. Bolt on the head, stick on the belts and rotate. Pull apart and measure flat bits. Squish 0.8mm is starting to get extreme. 1mm-1.2 nice. After that your just dropping off compression. Depends what you want. When playing with high lift or long duration cams, larger valves, valves can get stinky death close to the piston.

Golden rule: Contact is bad and will cost money.

Mr.R

I have to disagree with Griff here...
Anything over 1.00mm squish clearance and it isn't really doing anything at all IMO.
The squish needs to be 1.00mm or under to be doing what it's supposed to do.
I tend to use 0.8mm squish with steel rods.
0.8mm is pretty safe on a Desmoquatro, anything under that could be heading towards trouble.
Although some Ducati race teams used as low as 0.75mm without any problems.

Steve R

Ducal

Thanks guys all is now a lot more transparent

brad black

do you have 926 pistons maybe? i was supplied some alleged 955 pistons for a build that in reality were 926 i'd say, as i had to space the cylinders up.

Ducal

Brad ....... I would love a pair of 12220351A pistons for my 888's but for sure the ones fitted to the 916 are brand new 12220431A.

Please if you have a set PM me.

Mr.R

I've been down in my workshop this morning getting on with things but...
During a smoking break I was thinking of your question/problem and I think I've come up with the answer!

Early Desmoquatro motors (851,888,916 and 916SP's) had a very thick composite head gasket.
They were fibre gaskets with crushable steel rings (flame rings for the combustion chambers) around the areas need to be sealed.
After some failures Ducati redesigned the head gaskets, coming up with a thinner laminated Steel item, these were 1mm thinner than the earlier items.
This problem was over come when fitting the later type to earlier motors, by the fitting of a 1mm thick steel spacer under the barrels.
So I think the pistons hitting the heads was caused by fitting latter type/thinner head gaskets to your earlier motor, without using the thicker spacer/base gaskets needed.
Whoever built your motor before obviously had some thicker base gaskets made up, to negate the need for the 1mm spacers + some steel base shims.

Steve R

RWG

I just love this forum.
Even when I have no experiance or at times understanding of subject, it is the fact all this knowledge is shared.

Thanks
RWG

Ducal

Gav Forbes ( FME ) in Perth is making some at 1.6. He is incredible. Just made reversion tank fittings for me exact to OEM with a dedicated punch for the ball seat.