Barrel base compression plate

Hello all,

Has anyone used one of these?, RGM list a barrel base compression plate for the 750/850, at .030" thickness.

My old knee, not being what it used to be, needs all the help I can give it, so am quite keen to lower the compression ratio a bit, not worried about any loss of performance, would anyone know what compression ratio, from the standard spec, the above compression plate would give?,

Thanks,

John

I have used a compression plate on Atlas motors when using Commando pistons. It takes about .040 to drop the compression one point. [depending on where you are starting at]

Note that you will likely need to use larger jets in the carb when dropping the compression ratio.

The difference in kicking one over is very small. Jim

Remember to check valve rocker geometry with changes in engine stack height. I've come up with a exhaust valve lift compression release but ain't had time/money to get machinist to install yet. Best builders out there can not find a way to place release in bore like a chain saw nor in head like Harley s. No one sells the spark plug releases any more d/t them all blowing right out.

Here is a formula which can be used to either calculate the thickness required to achieve a desired CR, or to determine the CR that will result from a given thickness:

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

h = thickness of shim
s = stroke of engine
CR1 = known compression ratio before shimming
CR2 = compression ratio after shimming

Note: the formula is dimensionless - this means thickness of shim and stroke can be in inches, mm, or furlongs....just be consistant! Also consider that one gasket at cylinder base is the norm and using a shim will require two gaskets....the compressed thickness of the second gasket should be considered part of the overall shim thickness.

comnoz said:

The difference in kicking one over is very small. Jim

Click to expand...

This is what you need to hear. I have a compression plate that I took out. I used it, not for your reason, but to detune. I got over that.

Efforts might better be spent on technique and one kick solution like single carbs kit and high end ignition system.

Any pop that you reduce in the motor will be missed.

Mkv if your wanting to make starting easier I can recommend the Alton Ekit I know it's not really the answer to your question but I have one an they are worth the cost an you can start without troubling your knee.

Leave the CR the same or a bit higher and fit a hotter cam so intake closes later to shorten effective compression stroke. By far most my C'do life and times starting has been just a prime and one step down w/o an actual kick to start. Both on contact breakers or Boyah [with hot battery]. If not going ES then may be end of C'do riding w/o a helper. Compression releases would help the poor shagger gear too.

Thanks for the reply's all,

Interesting comments, I did wonder on the valve geometry Hobot, but figured as its a well known thing to do on British twins, or used to be, that it would be OK, I have never read of any warnings to say other wise, the hot cam is something I would never of thought of!, clever stuff.

Pete.v, although the Commando has been off the road for several years, I have owned the bike for, must be getting on for 25 years, so I think my technique is, or rather was!, not an issue, just wanted to make it easier, I think you are spot on with carburettor and ignition, from what I have read, the tri spark would make things a lot easier, I have had an old Boyer, was once a new boyer!, fitted for most of the time I have owned the bike, can kick back at times......

Am afraid Toppy that the Alton kit is just beyond my financial reach, it does look nice!!.

Thanks for the formula TexasSlick, I have not seen that one before, very handy indeed.

Interesting on using larger jets when lowering compression ratio, I have only lowered the compression ratio on one engine, a Reliant 850, which was fitted with an SU, so just a case of altering the jet height there, I know I did re tune, but did not take too much notice to be honest, thanks for your observation on the lowered Atlas compression ratio Jim, so even though it was a very small difference, it was noticeable then, thats good, but maybe sounds like you would not bother in my case?.

I think maybe I am leaning towards using a compression plate and fitting a tri spark ignition when I can afford it, also would like to go to a single carb, I did try a mk2 Amal kit from RGM a few years back, but had a lot of trouble with the slide sticking to the carb bore when shutting down, engine vacuum!!, I have not heard of that problem with single Mikuni kit, but thats another post I think!!.

Thanks all, food for thought!,

John

John, you are right its was/still is common practice to alter stack height w/o bothering to re-optimalize the rocker sweep on stems, just less wear and more hi rpm tolerance - unless going ape shit extreme then could bind break on first kick over. Set plug gap narrowner, get the trispark -set one red hair towards retard and get float level up so [cleared] pilot air jet likes a bit less than 1.5 turns out for nice idle w/o nil stumble on throttle snaps. Put in 15/40 diesel oil for less kicker drag and Bob will be a favorable hi placed relative. Amals get tickled to just wet and leave throttle closed till fires, Miki carb turn enricher on, leave throttle closed then try to catch rpms with low opening till can get off enricher. A dash of acetone helps cold gas vaporize more too. I've been spoiled for long periods with two Combats and one P!! of expecting one step on starts to point if it didn't I didn't step on again till checking key and fuel or battery or coil or plug wires off etc.