Mystery 500cc Cam

I recently acquired an incomplete Model 7 (1952) bottom end from a lot of parts that I bought. After inspecting it, I am pretty certain that this was professionally rebuilt at some point. The cases have been vapor blasted, new bushes and bearings have been installed, and there it some work that has been done to the case. I am not sure exactly what I am working with, so here are some pictures.


1. Firstly, the crankshaft looks to have either been lightened or balanced as it looks like a block of Swiss Cheese. I also noticed a ''0.20" stamped into, so I assume the journals have been machined. The crank has a stroke of 72.6mm, which matches that of the 500cc twin.

2. Second, the camshaft looks to be brand new and I cannot find a cast or part number on it. Does anyone know what the cam specs for a 500cc twin are? That would be appreciated as I do not believe this is the stock cam. It you look closely, you can also see that the cases have been cut around the cam (the sharpie is still present haha).

3. Third, the pistons that came with the engine have damage to the skirts. I searched for "GPM 6812 6822" in Google and it returned results for 600/650cc pistons. I measured them and they are in fact 68mm in diameter. I am thinking someone machined the barrels to accept the 600/650cc pistons and kept the 500cc stroke

I'd like to put the motor back together and have found a few barrels on Ebay. My question is: this cam does not have markings and I am guessing it is a performance part because the casing was shaved to make room for it. I have a few flat lifters for a 750cc Atlas, but the early 500cc lifter were radiused. Does anyone know what the 500cc cam specs are?

Cam specs:
http://atlanticgreen.com/camsurvey.htm

Long skirt 99 high comp pistons have been hit by rods or case mouth , I would try some short skirt 650 SS or manxman pistons , Cam has much more lift and duration than the std or even SS cam. Could be a nippy motor while it hangs together. Valve gear will need to be better than std M7. Engine is now probably 525 cc , Its been done before . Rods may be the weakest point although the 88 stroke is a better bet than the 99 's. GPM dont seem to understand the difference between the motors. Will need 10 thou or more valve clearances.

norton bob said:

Cam has much more lift and duration than the std or even SS cam. Could be a nippy motor while it hangs together

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I know it's hard to tell, but do you think I would need to run flat or radiused followers?

Flat .

The crankshaft would scare me! Heard toooo many stories about them cracking and blowing up. Just my two cents Good luck!

mean gene said:

The crankshaft would scare me! Heard toooo many stories about them cracking and blowing up. Just my two cents Good luck!

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With what looks like the lumps on that cam, I agree. Especially ground -.020". They need all the metal they can get.

If you can get the internal stress raisers in the crank removed it will survive longer , Ok for classic road use at sensible revs.

norton bob said:

If you can get the internal stress raisers in the crank removed it will survive longer , Ok for classic road use at sensible revs.

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I haven't heard of that.....what are those?

norton bob said:

Long skirt 99 high comp pistons have been hit by rods or case mouth , I would try some short skirt 650 SS or manxman pistons , Cam has much more lift and duration than the std or even SS cam. Could be a nippy motor while it hangs together. Valve gear will need to be better than std M7. Engine is now probably 525 cc , Its been done before . Rods may be the weakest point although the 88 stroke is a better bet than the 99 's. GPM dont seem to understand the difference between the motors. Will need 10 thou or more valve clearances.

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Do you have a good source for those pistons? I've been having trouble tracking them down.

The new pistons these days are a bit heavier than the old, so depends what you want out of the bike. Best case is like @norton bob says, an old (or potentially NOS) pair. You don't yet have cylinders? I've been collecting 500 stuff for a while, so I'd be happy to see if I've got a pair of orphaned pistons, once you figure out what size you need.

500cc cylinders are pretty tough to come by since there have never been new ones made since production ceased. Finding a good head might also be a sticking point. The SS downdraft heads are tough to come by that haven't had their ports butchered beyond what a 500 can use. A standard 7/88/99 head will not provide enough flow for that cam, unless well modified. The Atlantic Green site has a load of great info for the heads as well as the cams.

The 500cc 88 crank is notoriously fragile if you're flogging it, while the 88SS cranks are better (thicker wall at the big end journals), but much harder to find for that reason.

Did the bottom end come with the 9 fin barrels in the background, or are those the barrels from the Atlas (where the followers you mentioned came from)? If those are 650 barrels, it might be easier to go with those, and get a big journal 650/750 crank, rods, and 650 case, if you want anything resembling the hot rod that that cam is pointed toward. The flywheel is different between the 650 and 750, but all that might be easier to find than good 500 barrels these days. You might even have most of that stuff in that lot of parts.

I called the guy I bought the bike from and asked him if he might have the barrels and head lying around. Sure enough, he did and I picked them up today. The heads looks to be rebuilt with new valves with what I believe are are titanium valve retainers....all 500cc stuff. The barrel is in great shape and has the radiused followers.

I will keep an eye out for pistons. Are the stocks rods known to be fragile?

Fritzer_108 said:

Are the stocks rods known to be fragile?

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Great news!

Not as fragile as the cranks are known to be, but age can be an issue with any of this stuff. A Model 7 is old.

New rods are available from Andover Norton.

I forgot to mention...the rockers in the head are missing. Are they interchangeable with later twin models?

Fritzer_108 said:

I forgot to mention...the rockers in the head are missing. Are they interchangeable with later twin models?

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There were a lot of changes up there, including rockers. What works with what will probably be dependent on identifying that cam. I don't remember quite when the rockers changed geometry, but figuring out what that cam is (or close to) may point in some direction of contemporary rocker use.

The spindles are also pretty picky. Some are scrolled for oiling, while others are smooth with flats, based on your oil pump, 3 or 6 start (return puke vs pressurized). If there's an oil pump and pinion in place that would help determine some of this stuff, but not all. Head must be heated to push/pull spindles in and out.

One more bit of esoterica to be aware of for your motor. The older cylinders' pushrod tunnels were relatively narrow, so when you go to a hotter cam, and use the larger diameter aluminum pushrods, they clashed against the inside of the tunnel. There are 3/8" chromoly rods available, but you still may have to open up the tunnels a bit.

t ingermanson said:

One more bit of esoterica to be aware of for your motor. The older cylinders' pushrod tunnels were relatively narrow, so when you go to a hotter cam, and use the larger diameter aluminum pushrods, they clashed against the inside of the tunnel. There are 3/8" chromoly rods available, but you still may have to open up the tunnels a bit.

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I noticed that the tunnels on this cylinder seemed to have been cut and that would explain why. After checking the casting numbers and a few dimensions on the head, I was able to determine it's a 88SS downdraught head.

Fritzer_108 said:

I noticed that the tunnels on this cylinder seemed to have been cut and that would explain why. After checking the casting numbers and a few dimensions on the head, I was able to determine it's a 88SS downdraught head.

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That's a sought after head. I hate to harp on it, but those early dynamo cranks are suspect with all the performance bits added. Be careful.

Fritzer_108 said:

I noticed that the tunnels on this cylinder seemed to have been cut and that would explain why. After checking the casting numbers and a few dimensions on the head, I was able to determine it's a 88SS downdraught head.

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If it really is an 88SS head in good shape they are worth gold.
Back in racing days you would be happy to pay NZ $1500 for a real 88 SS head.

If your getting work done on it only the best will do.

But it's strange it is with generator cases. Those heads are 1962/63 onward.

RGM are advertising 500 twin pistons but I imagine they will be low compression