De compression plate

Hi

How thick can you make a decompression plate? Just having a quick look at the new pistons in the 750 race bike & they appear to be about 2.37mm higher than the old ones. (sorry metric vernieer guade)

I will take the pistons out & measure them up against the old ones but lots of bits to look at. Machining the tops of the pistons for squish, re doing the valve pockets. Checking the height of the barrels. Not to mention the thickness of the crowns of the new pistons.

Photos to follow.

all the best Chris

Chris said:

How thick can you make a decompression plate?

Click to expand...

Engines in the past have had some quite thick decompression plates,
but presumeably you are interested specifically in Commando ones.

2 or 2.5 mm doesn't sound out of the question. (?)
The limiting factors are probably
a) how tall the crankcase studs are, and
b) how long the pushrods are.

Finding another method to reduce their height sounds the better option though,
since a plate won't improve the engines strength,
and cutting out such a plate is going to be a fair bit of detailed work.
Laser cutting off a base gasket as a template would be a quik-n-dirty way, IF you go ahead with this.
You'll need 2 gaskets to make sure it seals too, allow for their thicknesses.

The one on the shelf that I once detuned a Combat head with is .030"(.76mm). I think with a gasket on both sides brought it to .040". Anything more than that and adjustments will have to be made in the valve train.

Another key issue is honing any step that may be present in the cylinder whether on top when taking one out or on the bottom of the stroke when adding one.

A formula to calculate the shim thickness to obtain a desired compression ratio is as follows:

h/s = (CR1 - CR2)/{(CR1-1)(CR2-1)}

where h = height of shim which includes additional gasket thickness
s= stroke of engine
CR1 = compression ratio without shim, and CR2 = compression ratio with shim.

Note that the formula can be worked to calculate the resulting CR for any given shim thickness.

If you want to split hairs, allow 10 - 20 % reduction in gasket thickness for gasket compression. Two gaskets are required with a shim, only one with no shim. The formula assumes one gasket is present and the second is part of the shim thickness.

Be sure to measure shim height, gasket thickness, and stroke in consistent units, either all metric or all inches.

Slick

With really thick base plates, there's also the question of whether the tappets have enough drop. Early 750 engines had different tappet separation plates that didn't allow as much travel as the later design, and sometimes required grinding for more drop to allow proper clearance with race cams with smaller base circles. I assume you would eventually run into the same problem with the later tappet separation plates if you used too thick a base plate. I've used up to .040" base gasket/spacer in an 850 with race cam with no problems, but don't know how much further one could go. You might also be able to profile the tappet separation plates for more drop, if required. I've never taken the time to actually measure the limits, but maybe someone else here has. As others have pointed out, you would also at some point need longer pushrods.

Ken