500 twin cranks

Peteyoung75 said:

I only wanted to check my crank

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That’s how it starts mate …

Fast Eddie said:

That’s how it starts mate …

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You gone down the fancy parts route?

Peteyoung75 said:

You gone down the fancy parts route?

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A few times !

Fast Eddie said:

The advantage to the smaller engines is that Jim offers ‘ultra light’ pistons and rods.

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Being 6.0" long c/c (*), rods for the 500/600 twins are NOT identical to the 650/750/850 engines (5.875" c/c). I can't find direct replacement rods within Jim's offerings, or am I missing something?

If a machined cylinder barrel is being considered, it may be shortened such that the standard length (lightweight) rod (5.875" c/c length) can be used. This will give a rod/stroke ratio of 2.06 .

Use of the ultra-light rods proposed by Jim (6.4" c/c) yields a rod/stroke ratio of 2.245 . It's an impractical choice as it would require a very tall barrel. I suggest to keep rod length ratios between 2 and 2.1,
which seems like a good compromise between TDC dwell and the desired piston acceleration.

* https://www.accessnorton.com/NortonCommando/rod-ratios.40323/

- Knut

Get a second opinion if you're going to run that dynamo crank. The "just put locktite on it" to fix the undersized/scarred main journal is dodgy and would put up my guard. I've pulled that bodge and it doesn't turn out well. There's better (and relatively cheap) ways to fix it, and now's the time to do it.

The SS cranks with the smaller diameter sludge trap still broke on the drive side, but less frequently than the standard 88 cranks. The reason why subsequent cranks had larger big end journals, and why no aftermarket crank is built with the small journals.

These were hotshot bikes at one point in history, and were ridden by the gamut of riders. Riding with mechanical sympathy will get you down the road at 70 all day. Just don't drop to 3rd to pass at 80, unless you're real smooth.

Those new alloy barrels will bolt right up when the time is right, but don't forget the JS hot cam, BSA lifter kit, and beehive springs. Can't short change yourself now. You're in too deep!

t ingermanson said:

Get a second opinion if you're going to run that dynamo crank. The "just put locktite on it" to fix the undersized/scarred main journal is dodgy and would put up my guard. I've pulled that bodge and it doesn't turn out well. There's better (and relatively cheap) ways to fix it, and now's the time to do it.

The SS cranks with the smaller diameter sludge trap still broke on the drive side, but less frequently than the standard 88 cranks. The reason why subsequent cranks had larger big end journals, and why no aftermarket crank is built with the small journals.

These were hotshot bikes at one point in history, and were ridden by the gamut of riders. Riding with mechanical sympathy will get you down the road at 70 all day. Just don't drop to 3rd to pass at 80, unless you're real smooth.

Those new alloy barrels will bolt right up when the time is right, but don't forget the JS hot cam, BSA lifter kit, and beehive springs. Can't short change yourself now. You're in too deep!

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Yeah I think I’ll definitely pull it apart again. As I’ve got closer to finishing the assembly ((currently bushing rocker spindles) I’m feeling more keen to go back over it.

I’m getting tempted to get some fancy parts now though

Talk to Jim Schmidt.

As Knut said, it’s certainly not clear that the ‘usual’ JS long rod approach would be ideal on your short stroke.

Perhaps his ultra light pistons designed for stock rods would be a sensible option ?

But I don’t know what Jim has / has not already done on 500s.

I can tell you that his long rod / light pistons work fantastically in 850, 920 and 1007 motors !

I’ll have a think. Feel like I’ve already been a bit of a time waster as I’ve previously made enquiries about JS pistons and also the JS0 cam and followers kit.

As someone who has a 88SS project on the (very) slow burn, and waffled a million times back and forth, I see the benefit you'll get from the long rod and light piston for the 500 is more limited than what you'd see with the longer stroke motors. The parts are undoubtedly an upgrade from stock, or the Thunder/Gandini/JP option. But,,, we're starting with a motor with a bhp in the low-to-mid thirties, and you might get it up to 45(ish?) with those parts, new inlet tracts, exhaust, and careful rebuilding, all the while rubbing up against the ability of that crank to deal with the upgrades. A Norton 88 is never going to set the world on fire with its power, but one of its natural-born characteristics is its already relative smoothness, thereby allowing the rider to maintain the revs comfortably.

If you're racing (you didn't mention if you were), then there's no end (or bottom), so go for it! If you just like to hotrod stuff and want to develop the bike for fun, there's no harm in it, so go for it! If you're just looking for a good, solid cruiser, the very nice parts aren't going to make or break an enjoyable bike or enjoyable times on it. You might instead spend the money on suspension upgrades that will make the most of what you've got (if you haven't already, of course).

Feel more than free to ignore me, but it's just what ran through my mind when debating which parts to build mine with.

Peteyoung75 said:

I’m getting tempted to get some fancy parts now though

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Molnar barrels, JS long rods and little JS pistons would save some weight and the rods and pistons would reduce reciprocating mass if stuck with the stock weight 500 crank. The JS0 is basically a stock cam with smooth ramps. You need to bump it up 1 or 2 numbers if you want a hotter cam. The stock springs and radiused stock tappets as well as stock length pushrods would suffice for a JS0 and JS1. If you go JS2 you might want the JS beehive springs or some KW race springs. If you shave the head use a thinner head gasket and don't use a base gasket you'll need shorter pushrods to improve rocker to valve tip geometry.

Just passing information along learned when rebuilding my 750 engine 3 times with fancy parts. The info might save you a couple of pounds and may or may not float your or anyone else's boat. As t ingermanson says feel free to ignore me, and I'll add in my case everybody else does unless they are looking for someone to tell they are wrong. lol

I’m not building this to race - I just want to know I can confidently open it up on the road without (too much) fear of it blowing up.
The crank has been on my mind since I first got the engine apart.
I need to also bear in mind that I’m not rich and that I also have my BSA C15 trials in bits
I’ve thought about getting this engine up and running pretty much as is (other than rechecking the crank) and getting on with other bits and pieces on the bike over time (I’d not had it long before taking it too pieces). This would get me a chance to get to know the bike better and how I want to use it although I’d have to be a bit restrained. It would also pass the 12 months or so until Andy Molnar crank should be available. Once I’ve got a decent crank I guess the other concerns regarding piston and rod weight will be much reduced.
It’s a bit of an oddball bike. Dynamo bottom end and barrels, SS head, BTH electronic mag. So someone’s spent some money on it but it was in a bit of a state. Both the barrels and head needed a skim. The tapper adjusters had already been digging into the valve tips so I wonder if the rocker geometry was already off. If so it’s going to be worse now.
I think I’ll do the following -

Finish bushing the rocker spindles (don’t hold your breath)
Assemble and fit head
Check valve to piston clearance
Check combustion chamber volume and work out CR
Check valve and pushrod geometry.
Write a new list including taking the bottom end apart and having another look at the crank

Maybe once I’ve got that sorted Andy will have a crank!

Some great input in here - thanks all

Have a read through this for a bit of inspiration on ‘hopping up’ a 500…