Pushrod technology

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dguzzi

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Jan 28, 2017
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I searched and read a bunch but..... I'd like to know if its accepted to Loctite the loose ends? Or is there new and better wonderful pushrods available?
This being a Combat build. Is it necessary to adjust length to achieve a "best case" scenario? Thank you!
 
I always use Loctite 620 cylindrical retaining compound when making custom pushrods.

I make the pushrods for the Combat engine at .040" shorter than for the standard Commando, but the correct length will also depend on other non-original mods, like different thickness base and head gaskets, or different length valves.

No real need for better pushrods for most builds. The stock Commando rods work fine, and are fairly easy to shorten.

Ken
 
I should have mentioned the 620, its what I would use! Nice to know about .040, I'll check into that but I don't really have a good accurate way to do it. Thank you for the response, its good info.
 
Lots more discussion of Commando pushrod tech in older posts. This is just one of them:

Commando Pushrod Tech

Time for a new tech thread. A couple years ago I bought some basic tooling from Comp Cams for making pushrods, and have been making custom length Commando pushrods to order. I've so far made them in 2024 T3 aluminum tube, in either .058" wall or .090" wall. Stock Commando pusrods are .062"...
www.accessnorton.com www.accessnorton.com

Ken
 
Also more discussion available about correct Combat pushrod length in older posts. Varies from .036" shorter to .050" shorter, and even more if higher lift cams are installed. Look for posts from Dyno Dave on the subject.

Ken
 
dguzzi said:
I should have mentioned the 620, its what I would use! Nice to know about .040, I'll check into that but I don't really have a good accurate way to do it. Thank you for the response, its good info.
Click to expand...
Just a consideration…with .040” milled from the head it’s nice to get .060” shorter pushrods to get the rocker geometry back to normal. Norton didn’t do this and combat spec engines tend to wear valve guides as a result.
Pushrod technology
 
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How to check if the head I'm using has been milled? Measure the squish area depth?
Also thank you for the link, I didn't see that during my search.
 
Hopefully LAB will chime in on squish band depth but he most likely will need to know what number is stamped on the head. You can always PM him
 
If you built a 750cc Commando motor with a billet crank and the stroke shortened to suit the 850 (79mm) bore size, you would probably be able to rev it to 9000 RPM., and get more power. You would need valve gear which would be able to keep up. But most Commando motors can only be allowed to rev to 7000 RPM, so valve gear is not an issue. An Atlas motor can be revved regularly to 8000 RPM without tangling valves.
 
dguzzi said:
How to check if the head I'm using has been milled? Measure the squish area depth?
Also thank you for the link, I didn't see that during my search.
Click to expand...
If the head is on the cylinders there is a noticeable difference in fin spacing at the head to cylinder junction.
 
acotrel said:
If you built a 750cc Commando motor with a billet crank and the stroke shortened to suit the 850 (79mm) bore size, you would probably be able to rev it to 9000 RPM., and get more power. You would need valve gear which would be able to keep up. But most Commando motors can only be allowed to rev to 7000 RPM, so valve gear is not an issue. An Atlas motor can be revved regularly to 8000 RPM without tangling valves.
Click to expand...
That'd be the ventilated atlas cases then? :D
I used to own a set and I suspect many others did too
 
baz said:
That'd be the ventilated atlas cases then? :D
I used to own a set and I suspect many others did too
Click to expand...
Just for information, Al, the factory short stroke 750 motor used the standard 850 Commando bore of 77 mm with a stroke of 80.4 mm (standard Commando crank forging ground to shorter stroke). It came with 10.5 compression ratio, steel connecting rods, big valve head, and 4S cam. The factory red-line for this engine was 7500 rpm. With an open megaphone exhaust and 33 mm carbs, they claimed 80 bhp (at the crankshaft) at 8,000 rpm. I raced with one of them for several years, and if I could have run it to 9,000 rpm without valve float (which I couldn't), the power would have dropped well below it's peak, which was a bit below 7500 rpm. But you wouldn't have to worry about it long, because the engine wouldn't have stayed together very long under that abuse. Even with a 7500 rpm red-line, I managed to crack one crankshaft and eventually break one of the connecting rods. I switched to a billet crankshaft and Carrillo rods and had no more failures, but it still wasn't a 9,000 rpm engine.

Ken
 
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ILLF8ED said:
Just a consideration…with .040” milled from the head it’s nice to get .060” shorter pushrods to get the rocker geometry back to normal. Norton didn’t do this and combat spec engines tend to wear valve guides as a result.
View attachment 101452
Click to expand...

Yeah, it's a shame Alloy-Tech is no longer in business. They supplied several really good items, including a ball-bearing points breaker plate using high quality bendix points, with an Oldham coupling to the end of the cam. I really liked that as a racing ignition system back in the '70s.

However, their claim that their 4130 chrome-moly pushrods are lighter than stock is totally false. The stock Commando alloy pushrod (the longer one of the pair) typically weighs 29.7 grams. The Alloy-Tech 4130 rod with hardened steel ends typically weights a bit over 50 grams. They also made alloy pushrods with either hardened steel ends, which weighed 29.8 grams, or with hard anodized aluminum ends, which weighed 22.7 grams. Unfortunately, my experience with the ones with aluminum ends was that the rocker arm ball eventually wore through the hard anodizing, and then wore quite rapidly.

Ken
 
I did my first Combat by trial and error. As already said, there are so many variables at play it's impossible to have a definitive answer.
Starting at 0'040" is favourite, but I had to go a little further with mine.
It sort of crossed the line between engine building and race preparation. OK once you get into it.
Visualising the issue was easy enough on mine: the tappets had left witness marks on the valve tips which started in the middle and reached to the edge. Not good
 
dguzzi said:
Would you guys know the best length and how you are measuring it?
Click to expand...
All the measurements I posted are overall length, from the end of the ball to the top of the cup. That's how Norton specified pushrod lengths, and it's easy to measure, so that's what I used. If making pushrods with different ends (I used ends from Smith Brothers and Compcams), it's useful to measure the depth of the cup using a 5/16" ball, and correcting overall lengths accordingly. The difference is typically only .005 - .010".

Ken
 
lcrken said:
All the measurements I posted are overall length, from the end of the ball to the top of the cup. That's how Norton specified pushrod lengths, and it's easy to measure, so that's what I used. If making pushrods with different ends (I used ends from Smith Brothers and Compcams), it's useful to measure the depth of the cup using a 5/16" ball, and correcting overall lengths accordingly. The difference is typically only .005 - .010".

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

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