Atlas with JS rods & pistons?

Matchless

Matchless

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This coming year I am planning to rebuild the engine on my Atlas Domiracer, using JS long rods & pistons. Just wondering if any of you chaps had gone down this route. I have Jims bits in my 920, & that is a fantastic engine, but as it is rubber mounted I don't know how much shaking they have tamed. The Atlas is for fast road use & uses the standard slimline frame. Just looking for some independent reviews before commiting my hard earned cash.

Martyn.
 
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No experience, but you get my vote !

According to Jim you can use ultra light rods in that motor. The (comparatively) small pistons will also be very light.

With good balancing I think you’ll have a super sweet motor there mate.
 
I have the long rods and lightweight pistons in my P11 engine (essentially an Atlas engine), but haven't gotten it started yet!! @Schwany has run his, however and seems pleased with it.
 
A rigid mounted Norton twin engine will in my experience never be as smooth as a rubber mounted engine on the street and cruising, unless the rubber mounts are shot. However, if your Atlas crank has never been re-balanced from what came out of the factory, getting it balanced for the weight information Jim provides for the rods and pistons he sells it should be one heck of an improvement.

Where I think Jim's long rod conversion parts shine for me is when the engine is getting close to or hoovering at max HP. At around 7000RPM my engine feels smooth strong and like it is just getting started. Your performance will depend on a lot of other factors though. I have a lot more in my 750 P11 engine than his long rods and pistons. If just making the long rods and pistons change, I think it will smooth out some as long as the crank is rebalanced. Basically, what I said in that paragraph above and what Nigel said earlier.

Andy Molnar uses 75% BF for a rigid mount. Jim is similar for a rigid mount maybe 72%. Ask him.

What other changes do you have planned for the motor when doing the rods and pistons?
 
Schwany said:
A rigid mounted Norton twin engine will in my experience never be as smooth as a rubber mounted engine on the street and cruising, unless the rubber mounts are shot. However, if your Atlas crank has never been re-balanced from what came out of the factory, getting it balanced for the weight information Jim provides for the rods and pistons he sells it should be one heck of an improvement.

Where I think Jim's long rod conversion parts shine for me is when the engine is getting close to or hoovering at max HP. At around 7000RPM my engine feels smooth strong and like it is just getting started. Your performance will depend on a lot of other factors though. I have a lot more in my 750 P11 engine than his long rods and pistons. If just making the long rods and pistons change, I think it will smooth out some as long as the crank is rebalanced. Basically, what I said in that paragraph above and what Nigel said earlier.

Andy Molnar uses 75% BF for a rigid mount. Jim is similar for a rigid mount maybe 72%. Ask him.

What other changes do you have planned for the motor when doing the rods and pistons?
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The crank will definitely be dynamically balanced to suit. Final engine spec not yet decided, but will probably use a PW3 cam & a cr of 9.5 or 10:1. I. I'll be fitting the spare Maney crank & barrels I have, & might try the stage one head which I removed from my 750 Commando. On the other hand I might just use the standard Atlas head.
 
Matchless said:
The crank will definitely be dynamically balanced to suit. Final engine spec not yet decided, but will probably use a PW3 cam & a cr of 9.5 or 10:1. I. I'll be fitting the spare Maney crank & barrels I have, & might try the stage one head which I removed from my 750 Commando. On the other hand I might just use the standard Atlas head.
Click to expand...
It'd be great if you get the time to try both heads for comparison
 
Matchless said:
The crank will definitely be dynamically balanced to suit. Final engine spec not yet decided, but will probably use a PW3 cam & a cr of 9.5 or 10:1. I. I'll be fitting the spare Maney crank & barrels I have, & might try the stage one head which I removed from my 750 Commando. On the other hand I might just use the standard Atlas head.
Click to expand...
Wow, light barrels and light, strong Maney crank, stage 1 head, sounds fantastic !!
 
baz said:
It'd be great if you get the time to try both heads for comparison
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I've done that in the past with a stock Commando head and a Fred Barlow ported P11 head on my motor, and there is no comparison if top end performance is the goal. I was actually disappointed with the stock Commando head, which is why I took it off and have kept it for 30+ years. I was a lot younger when I did the head swapping though. A stock Commando head might be less disappointing now. ;)
 
Schwany said:
I've done that in the past with a stock Commando head and a Fred Barlow ported P11 head on my motor, and there is no comparison if top end performance is the goal. I was actually disappointed with the stock Commando head, which is why I took it off and have kept it for 30+ years. I was a lot younger when I did the head swapping though. A stock Commando head might be less disappointing now. ;)
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What type of stock commando head was it ?
IE 28mm inlet etc?
 
baz said:
What type of stock commando head was it ?
IE 28mm inlet etc?
Click to expand...

Early 750 Commando and the inlet ports are smaller than my P11 head intake ports which are 31mm. I'd go out and measure them but there is a solid sheet of ice covering the walkway out to my garage. Last thing I want to do right now is end up on my ass on ice. They are probably 28mm though.
 
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The Maney head is the one I tried on my 750 Commando, but as it has 34mm inlet ports made the bike rather gutless at lower engine speeds. This may have been in part to the 9:1 cr I was running, but it would be nice to experiment with it on the Atlas. If I do & find I still don't care for it, then it will be up for sale. It would probably work better with 11:1 cr as discussed in a previous post, but as I have to use what ever petrol is available when I need it, this I feel certain would become a problem.
 
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And if the slimline frame has the room behind the head, you can make up a longer set of intake manifolds for 35mm FCRs. The right length manifolds have been known to make some usable power. That may not be of interest though if trying to maintain a period correct look.
 
Not too bothered about period look as the bike has disc brakes front & rear. As or FCRs, if by this time next year I make like Delboy & become a miwyunare, then I might consider buying a pair. 🤣
Appologies to readers from overseas who may not have seen Only fools & horses.
 
I have the Ultralight kit for a 650ss engine. Haven't built it up yet.

If done again, I would probably go for the lower expension 4032 billet pistons he sells instead of the JEs.

Lightweight billet pistons

Having a try at Billet pistons made from low expansion 4032 alloy (allows sightly tighter installation clearance than 2618 alloy). The advantage of billet is that you are not dependent on an existing forged shape. So you can have them designed anyway you want. The result in this case is the...
www.accessnorton.com www.accessnorton.com
 
If I were going to use stock rods and build a higher CR 750 Norton motor, I'd get the MAP Cycle 10:1 pistons.

Anyone chomping at the bit to share a horror story about MAP Cycle pistons?
 
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