Atlas engine weaknesses

baz

baz

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Having seen many blown up atlas engines and crankcases with patches welded up etc etc
I was wondering what the weakness was?
Is it the crank or rods etc
Or is the bottom end the same as a commando?
I'm asking as I have an atlas engine I'm planning to use and the one I have has flat topped pistons so it's basically commando engine spec
 
The Atlas has the same bottom end with the large sump plug but the top end is lower compression so it should be less stressed than a Commando.

There is a difference with the worm gear on the pump where there was a change from 3 start to 6 start during Atlas production which doubled the pump revs for the same crank revs.

Thttps://www.accessnorton.com/NortonCommando/difference-between-3-start-and-6-start-oil-pump.26985/page-2

So if you can look at the engine number to see if its pre/post the change then that could give a clue.
 
kommando said:
The Atlas has the same bottom end with the large sump plug but the top end is lower compression so it should be less stressed than a Commando.

There is a difference with the worm gear on the pump where there was a change from 3 start to 6 start during Atlas production which doubled the pump revs for the same crank revs.

Thttps://www.accessnorton.com/NortonCommando/difference-between-3-start-and-6-start-oil-pump.26985/page-2

So if you can look at the engine number to see if its pre/post the change then that could give a clue.
Click to expand...
I believe my motor has the 6 start oil pump
I'd prefer the dished low compression pistons but it's been rebored and I don't believe the low compression pistons are available in an oversize
I had thought the bottom end was the same as commando apart from things like the alternator keyway on the crank and balance factor
 
Yes , of course , that's the only picture I could get (from NOC uk ) ; the last one on the right show the main difference , near the bottom down v shape for Cdo and late atlas .
 
baz said:
I believe my motor has the 6 start oil pump
I'd prefer the dished low compression pistons but it's been rebored and I don't believe the low compression pistons are available in an oversize
I had thought the bottom end was the same as commando apart from things like the alternator keyway on the crank and balance factor
Click to expand...
About 3 years ago, I found a set of .040 over NOS OEM dished 7.5:1 pistons on eBay for my N15, a very lucky find at $100. I think the Atlas is 7.6:1 though.
 
My P11 bottom end hasn't busted yet and it's been higher than stock CR since 1993 and ridden hard many times when I was younger. Not raced however.

Below is a 10:1 version. Those are JSM high CR +.040 pistons on Carrillo long rods.

Atlas engine weaknesses


This so called Atlas P11 engine saw some high RPM and stayed together. It's not stock inside or out though.
 
I'm too cheap to build anything special, but I love seeing it done here. Very cool. As to the original question, it's just a guess, but from my experience, these bikes weren't built to last under the abusive conditions that many dealt them. Take one example - external, often plastic oil lines to the head. Now go race them in the desert and drop the bike a few times. I suspect oil would start sprinkling the chaparral. I've opened up many engines and the 'Universal British Twins' are an anachronism vs the 'Universal Japanese Machines'. As I recall reading the British engineers saying about the Japanese bikes "We are sunk. These are diamonds and ours are coal." Or something like that. Hope no twisted pantaloons, just my observation.
 
gpzkat said:
I'm too cheap to build anything special, but I love seeing it done here. Very cool. As to the original question, it's just a guess, but from my experience, these bikes weren't built to last under the abusive conditions that many dealt them. Take one example - external, often plastic oil lines to the head. Now go race them in the desert and drop the bike a few times. I suspect oil would start sprinkling the chaparral. I've opened up many engines and the 'Universal British Twins' are an anachronism vs the 'Universal Japanese Machines'. As I recall reading the British engineers saying about the Japanese bikes "We are sunk. These are diamonds and ours are coal." Or something like that. Hope no twisted pantaloons, just my observation.
Click to expand...
They say Edward turner took a trip to Honda to see what they were up to in the early 60s
And came back visibly shocked
I can believe it
 
baz said:
Having seen many blown up atlas engines and crankcases with patches welded up etc etc
I was wondering what the weakness was?
Is it the crank or rods etc
Or is the bottom end the same as a commando?
I'm asking as I have an atlas engine I'm planning to use and the one I have has flat topped pistons so it's basically commando engine spec
Click to expand...
another weakness is the way the rider uses/misuses: over-revving, lack of maintenance, lack of mechanical sympathy, etc…..
 
jan nelder said:
another weakness is the way the rider uses/misuses: over-revving, lack of maintenance, lack of mechanical sympathy, etc…..
Click to expand...
I once bought an atlas with a blown engine
The only thing usable was the head
The owner had blown the engine when the throttle had jammed open
He said as he was reaching to turn it off the revs climbed and the motor blew
 
baz said:
I once bought an atlas with a blown engine
The only thing usable was the head
The owner had blown the engine when the throttle had jammed open
He said as he was reaching to turn it off the revs climbed and the motor blew
Click to expand...
Misfortune, and I am sure it has happened to owners of jap bikes as well. The P11 and some TR6C models had a kill button on the handlebar.

- Knut
 
jan nelder said:
another weakness is the way the rider uses/misuses: over-revving, lack of maintenance, lack of mechanical sympathy, etc…..
Click to expand...
Americans used to race bikes right out of the crate, ignoring running-in, and the fact that brit bikes require more maintenance than two-strokes those riders might have been used to.

- Knut
 
We pulled an old Atlas motor apart yesterday. It had .030 Commando pistons in it.
Abuse? Stupidity?
** The cases had two big welds at the rear just below the cylinders
** Up front there was another big weld approx 3" just below the cylinders.
** Another weld inside the case by the cam
** Both rods are snug on the crank
** The rings do not move freely, difficult to push back into the piston. There was no rust in the cylinder. Everything was well oiled.
** The left rod showed wear and tear the length of it.
** Blue silicone seal was used LIBERALLY everywhere.
** The cam chain is bow string tight.
That's all for now.
 
gpzkat said:
As to the original question, it's just a guess, but from my experience, these bikes weren't built to last under the abusive conditions that many dealt them. Take one example - external, often plastic oil lines to the head. Now go race them in the desert and drop the bike a few times. I suspect oil would start sprinkling the chaparral.
Click to expand...
The Hybrids (e.g., G15CS / N16CS) had brass oil lines to the head. They fractured regularly. AMC's twins had internal oilways to the heads. Much cleaner design and it ensured reliable oil supply. Velocette used external oil pipes as ornaments. Now, that is an anachronism!

- Knut
 
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