RGM 920 kit

Several years ago I purchased an RGM 920 kit with plans to fit it to my MK 3 850. But the bike runs really well as is so I have left the kit on the shelf until now.
Not long ago, I learned from Jim Comstock that the 81mm 920 kits are perhaps not a great idea for a Commando.
The liner ends up about sixty thou thick which is too thin and does not have a long life expectancy. So I figured the kit was a waste.
Before putting it out roadside for the Sally Ann to pickup, I had a bit of a look at it this AM.
Although labelled "920 kit", it is not. I recall that his early kits used a Ford Csoworth 81 mm piston, however Roger said they had stopped using those and now used another piston which had identical weight to stock Norton pistons, so no rebalancing required.
He did not mention that these are actually an oversize of an 80 mm piston and actually measure 80.5mm. This will give 906ccs,according to my figuring. Also, it appears that the liner thickness after boring will be about 90 thou, not overly thick, but the same as my 92mm Terry Prince Vincent liners, and those have been shown to work very well.
So perhaps the kit will be of some use, if not on this MK3, then perhaps on some future project. They are sturdy little pistons!

Glen

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What is the OD of your liners? Jim

On closer exam, the liner OD is 3.339". The pistons are a smidge under 80.5mm , so an 80.5 mm bore might work. 80.5mm translates to 3.169"
This should leave a wall of 3.339-3.169= .17" .17/2= .085", so not quite the ninety thou. This is assuming the OD of the liners can stay as is.

Glen

With a bore centerline of 3.435 that will leave .096 between the liners...

I wouldn't want too much interference fit. Jim

Hello Worntorn,

Remember, you have to enlarge the crank case 'throats" to accept the larger liners. The head will also need to be machined. A pain if you ain't got the gear or a good workshop nearby.

I fitted a 920 kit because the barrels were stuffed and it was the cheapest option.

Cheers

How do you like it? Have you put many miles on it?

Glen

Here is another photo showing the piston wall. Sturdy little things, definitely not light weight, tho just slightly lighter than original 850 pistons. I think Hobot could drive his John Deere LA over one of these and it would stay intact.

worntorn said:

Here is another photo showing the piston wall. Sturdy little things, definitely not light weight, tho just slightly lighter than original 850 pistons. I think Hobot could drive his John Deere LA over one of these and it would stay intact.

Click to expand...

Wow, is that a steel strut piston. I haven't seen one of them for a few years...

Nortiboy said:

Hello Worntorn,

Remember, you have to enlarge the crank case 'throats" to accept the larger liners. The head will also need to be machined. A pain if you ain't got the gear or a good workshop nearby.

I fitted a 920 kit because the barrels were stuffed and it was the cheapest option.

Cheers

Click to expand...

I don't think Roger intends for crank mouth machining to be required with this kit in general, however he notes that some small amount of crankcase mouth material removal may be required in some situations.
He suggests checking to see if there are any interference areas between the new liner skirt and the crankcase mouth, and if there are, do the removal with a small rotary grinder, finishing with a flap wheel and then 1200 grit wet and dry.
When machining the cylinders to accept the new liners, he notes that after boring and honing the cylinders to 3.343" there may still be thin pieces of the old skirt remaining, and that these must be removed. So the OD of his liners must be the same or smaller than the original 850 skirt.
He also suggests moving the new holes out from the centre line by ten thou each, no doubt to give more material in that skimpy centre section that Jim mentioned.
This alone would seem to indicate the need to remove crankcase mouth material at least from the outside half of the bores.
I suppose ten thou is a small enough amount that it could be removed with hand held power tools, however if needed, I would be inclined to mount the crankcase in the Mill and go at it with the Wolhaupter boring head, much easier and tidier.


Glen

My experience is that with the 3.325" OD liners I used to sleeve iron cylinders for 920 kits back in the '80s, some machining (or grinding) of the case mouth was always required to get the cylinders to fit. I originally did them by hand with a grinder, and later progressed to boring them on a mill. I don't think you can fit anyone's 920 cyinders without some machining. Maybe you could if the liners were stepped so that they were thinner below the base, but I would think that would be a good recipe for broken liner bits dropping into the crankcase, a regular occurence with Dunstall 810 kit liners.

Ken

worntorn said:

How do you like it? Have you put many miles on it?

Glen

Click to expand...

One of several projects not yet running. All together and should run later this year. 10:1 compression, stock cam and RH10 head for road bike.
Let you know how it goes.

Cheers

Mark

comnoz said:

Wow, is that a steel strut piston. I haven't seen one of them for a few years...

Click to expand...

I wasn't sure if that was a good wow or a bad wow, so I did a bit of reading on steel strut pistons. Seems they were patented in 1933 but had already been used quite successfully in the 1927 and on Chrysler Airflow. So they might just be an old enough design to work well in a Norton. The benefit of the steel strut is that it controls expansion and allows for a tighter piston fit than with regular cast pistons ( not hypereutectic) or forged pistons.
They are also in use in the current production Chrysler Hemi V8s as well as a number of Mitsubishi engines. So they just might be modern enough to work in a Norton!

Glen

worntorn said:

comnoz said:

Wow, is that a steel strut piston. I haven't seen one of them for a few years...

Click to expand...

I wasn't sure if that was a good wow or a bad wow, so I did a bit of reading on steel strut pistons. Seems they were patented in 1933 but had already been used quite successfully in the 1927 and on Chrysler Airflow. So they might just be an old enough design to work well in a Norton. The benefit of the steel strut is that it controls expansion and allows for a tighter piston fit than with regular cast pistons ( not hypereutectic) or forged pistons.
They are also in use in the current production Chrysler Hemi V8s as well as a number of Mitsubishi engines. So they just might be modern enough to work in a Norton!

Glen

Click to expand...

I have to say I didn't know anyone was still using them. I saw a lot of them in 50's and 60's engines but they seemed to go away with more modern designs. Most of the ones I have seen years ago were very heavy and some of them had a bad habit of loosing their skirts. Not inferring that there is anything wrong with the pistons you have. Jim