RGM 920

Keith1069 said:

You have to take the cylinder cc + combustion chamber cc/combustion chamber cc which gives about 8.5:1.

Click to expand...

But that doesn't take the head gasket thickness into account, as even adding a 1mm thick head gasket will reduce that comp. ratio down to just below 8:1, and a 2mm thick gasket reduces it to below 7.5:1.

Right, so it is 414+ 55 + hd gasket amount / 55+hd gasket amount.
I recall reading that a compressed copper head gasket is typically about .30 thick so this adds 4 CCs.

So that would figure 414+55+4 /55+4= 8.0 to one give or take. This is assuming that the stock pistons rise flush to the cylinder top. I cannot recall if they do.

In any case, the big bore kit should increase the stock compression by 11 percent with the piston height set the same as stock.

Apologies, yes I missed that, should have looked up my old calculations done when I reduced my Combat CR with comp plate under barrels.

I'm attempting a Steve Maney 920 with a milled out Combat fastory
early big port head and too high Compression ratio for pump gas.

Question1, how does one endure kick starting a 920?

Question2, would the larger lower ratio RGM 1st gear set ease the task?

Rather pleased not to read of 920 drive train failure alerts.

hobot

I'll bet the 920 engines are real muthers to kick!

My combat is tough enough for my 140#.

If those are the pistons that they supply then they are not forged, they are a cast , slotted automotive piston,with steel expansion struts .

How much has to be removed from the cyl to accommodate the liners? I'm sure they figured this out. But how durable has this proved? I've a mystery 850 on my 2a project in the basement. When I crack him open this winter if I find I have a lot of work ahead of me I might opt for this kit. No replacement for displacement. Just wondering if I'd be. Approaching no man's land w/regard to durability

Gday rgrigutis,
how much has to be removed from the barrels(?), well I ve had an RGM 920 kit fitted to my yet to be finished project. The barrels when machined resemble swiss cheese, being able see right through in some places.The machinist (Martin Adams, formerly of SERCO UK)who done this for me said "it was down right bloody scary when the tool broke through"! And yes you can see the sleeves from the outside if pointed out, a nice coat of VHT high gloss brake calipre paint covers it up beautifully though!
The cost of the kit is roughly $300 Australian and maching/fitting $700,all up $200 short of Steve Maneys 920 Alloy barrels.If I had my time over again Id go the Maney option as you get the added benfit of less weight.
You cant beat the feel and sound of big cubes!!
Rgds Foxy