Pushrod technology

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Interesting write-up. At least they are up-front about the steel pushrods being heavier.


Looking back through I don't see overall length mentioned; just the amount to shorten. Can you direct me?

You're right. I didn't actually post any lengths. From the Commando service manuals they are 8.166/8.130 in. for intakes and 7.321/7.285 in. for exhausts.

Edit - Just to be clear, that's the lengths for the standard Commando engine.

Ken
 
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It interests me that no matter how many times you are shown to be wrong Alcotrel you never respond??
I never said it was. The Atlas could have been made short stroke. It would probably have revved higher and developed more power, because of the higher revs.
 
I never said it was. The Atlas could have been made short stroke. It would probably have revved higher and developed more power, because of the higher revs.
With a 79mm bore size, a 750 would have a shorter stroke. Engine sizes for road bikes, are often based upon what qualifies for road-racing classes. In the 70s, there was a 750cc class for competition between the UK and America. You will find that PW's race bikes were all 750s.
 
I never said it was. The Atlas could have been made short stroke. It would probably have revved higher and developed more power, because of the higher revs.
You were talking about a short stroke motor
You stated most commando engines can only be allowed to rev to 7000 rpm but an atlas can be revved to 8000 rpm without tangling the valves !
So this atlas engine with the weaker barrel and the 5/16" outer studs holding the head down with the weaker crankcase (weaker than a later 750 commando and weaker than all of the 850s) etc can rev an extra 1000 rpm safely right?
 
Whatever floats your boat, I like the way my 850 pulls and sounds around 3,500 to 5,000rpm. So I'm not too bothered about 7,000 revs. I like that it's an evolution of earlier motors. However, having watched that Barbers /Design Centre interview video in one of the other topics, I am amazed at both how slow the engine development process proved to be, and how difficult they found understanding the consequences of changes they made to these motors. One of the interviewees implying the mk2 850 was where they started to get reliable performance. The Japanese factories were quicker to learn and adapt.

But, this has strayed from combat pushrods. Sorry about that.
 
Let us know how the BSA conversion works . Is it worth the money? I know they have been out there for a while Thanks
The conversion definitely works.
On mine the cam wear is zero. Last time I looked at it, it still looked new.
What benefits the reduced weight really has is difficult to say, personally, I believe in it.
As to whether it’s worth the cost, I guess that’s personal choice. But it also depends what you‘re starting with, if your stock cam, followers and pushrods are shot, there’s probably nowt in it.
It’s also a great option if your cam follower tunnels are worn (mine were shot).
 
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Just for interest only, I weighed a standard inlet pushrod and compared it to one from JS Motorsports. Standard was heavier at 45g and 46g and the JS ones were both 32g.
I was expecting it to be the other way round as they look stiffer. It's probably the longer steel ends on the standard ones that make them heavier.
View attachment 102559View attachment 102560
Edit: Of course the JS ones can only be used with different lifters (BSA type in my case) as they have cups at both ends.
Ken mentioned that stock Norton OEM pushrods are 29.7 grams for the long intake pushrod.
Is your stock Norton pushrod an OEM item? I couldn't find any new hollow aluminium pushrods for my 920. All new items were either solid aluminium or steel, both types heavier than the original equipment.
A friend managed to find a set of originals in good shape, so I used those




Thanks
Glen
 
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There‘s stock and there’s stock IIRC.

Weren‘t original ‘stock’ one’s hollow, and therefore light, but new ‘stock‘ ones are solid, and not so light ?

My guess is your ‘stock’ ones are solid.
 
The conversion definitely works.
On mine the cam wear is zero. Last time I looked at it, it still looked new.
What benefits the reduced weight really has is difficult to say, personally, I believe in it.
As to whether it’s worth the cost, I guess that’s personal choice. But it also depends what you‘re starting with, if your stock cam, followers and pushrods are shot, there’s probably nowt in it.
It’s also a great option if your cam follower tunnels are worn (mine were shot).
Bragging rights are important, they make you motorcycle go faster. In a race motor, I usually lighten and polish the valve gear, and sometimes I have used cams with a slower lift rate. An 850 Commando motor does not need any of that to do 7000 RPM safely for a long time. The 850 Commando motor is similar to an Atlas motor but won't cop as many revs because of the stupid crank balance factor. A standard Atlas is probably quicker then a standard Commando, but would never be as smooth when you are riding it slowly.
My crank balance factor is about 72%. The bike feels really good when it is at low revs because the imbalance creates an adrenalin rush. At high revs the motor is lovely. This stuff makes me laugh. When I was a kid, I had Triumph 650s which had high crank balance factors and race cams. My mate's neighbours were going to take up a petition against me.
Isolastics do not stop crankcase damage, when you rev an unmodified Commando engine often to 8000 RPM. The kid who gave me my spare crankcases found that out. Thery now have an alunimium plate welded to them on the drive side.
 
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Yes, original pushrods were hollow and very light. I found that they are no longer available, other than an NOS find. I imagine that is why Ken setup to make his own hollow pushrods?

Same with the original conrods, they are also very light, so I used them in the 920 as well. Not hollow obviously. Original Commando innards were really quite decent if 6500 is used as redline. I have to remind myself to use 6 k as power drops off from there.
I don't think the standard Atlas was faster than the standard 850 Commando. Other way round on all road tests, save the very late black cap bikes.
The Atlas 1/4 mile tests show about the same as the 650ss, around 14 to 14.5 seconds .
The 750 Commandos were mostly low 13 seconds with 1 notable 12.69. The free breathing 850s were in that high 12 second group.
These are all period test results back when the reviewers actually took the bikes to a dragstrip and put them through the paces.
Nowadays they just calculate based on manufacturers claimed weight and horsepower numbers.
Sorry, drifting off again here.

Glen
 
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When you compare a Commando with an Atlas, you are talking about two very different bikes. An Atlas is much closer to a race bike. I probably think differently to other people. The only time I think a motorcycle should be ridden slowly is when it is on wet grass. I don't ride on public roads these days, because I always like to keep passing the other traffic. That way, your life is in your own hands. But if you do that, you risk losing your licence, and in Australia, if you lose one licence you lose them all.

If I had a road going Commando, I might change the crank balance factor to make it better at high speeds. But other than that, the motor is quick enough for road use without performance mods. With a race bike handling and smooth power delivery are often more important than outright power. If you lose a bit in the straights, you can usually regain it in the corners, if your bike is tame and has excellent handling.
I raced my 500cc short stroke Triton against a guy who as a very good 650 Triton. I could never convincingly beat him With my bike, it was always a love-hate relationship. That guy rode my Seeley against me when the Seeley had a Laverda 750 motor in it. I blew him off down a straight after a couple of laps, but my drum front brake had heated up. As we approached the corner at the and of the front straight, I was in the lead. I touched the front brake to slow for the corner, and the front of the bike went down - I had over-braked in front of the Seeley. I knew I was in trouble. The guy just rode around the outside of me, popped in front and grabbed a handful of disc brake. I was a bit out of practice and did not get my hands off the bars quick enough. The tank slapper launched me. I went down the road on the top of my head, and by the time I reached the ripple in the bitumen, thankfully I was on my side. I copped a dislocated chromo-clavicular joint. My mate owned the Seeley Laverda, and sold it. I tracked it down after asbout 3 years and bought it, but I could not get the motor, so it became a Seeley Commando. If that guy had been on his own bike when he crashed me, there is no way he would ever have been able to ride around the ourside of me in a corner. I have ridden his 650 Triton and it is a very nice bike, but neutral in corners, my 500 Triton handled better. I would never tall him the reason that was the case. His Triton is now a 750 and several times he has asked me how many gears my Seeley 850 has. He thinks he is smart - he took an RZ350 Yamaha to Broadford and beat a well-known guy who was riding a Seeley 750. I don't race on that circuit. If I took my Seeley 850 there I would almost certainly crash it. But the day will probably come.
 
If I was using a very hot cam and high revs in my 850 engine, I would buy Jim Schmidt's BSA type cam followers. Also Wassell used to sell hollow tappet adjuster screws, and aluminium nuts. I have used the screws , but cur one third thickness off the steel locknuts. The screws are adjusted by an Allen key.
 
The conversion definitely works.
On mine the cam wear is zero. Last time I looked at it, it still looked new.
What benefits the reduced weight really has is difficult to say, personally, I believe in it.
As to whether it’s worth the cost, I guess that’s personal choice. But it also depends what you‘re starting with, if your stock cam, followers and pushrods are shot, there’s probably nowt in it.
It’s also a great option if your cam follower tunnels are worn (mine were shot).
it also usually involves JS beehive valve springs, which work even at lower spring forces, which all helps with cam wear. My 850 mk3 motor with that set up sure spins up very nicely.
 
Ken had it at 29.7 grams. Could that 3.3 grams difference be scale variation, about 10%?
Anyway, the stockers are plenty light for 6500 rpm and I suppose the heavier steel ones are also fine for these old low revving tractor motors.

Glen
 
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