Valve clearances | GTAMotorcycle.com

Valve clearances

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bitzz

Well-known member
I'm bored. Let's see if I can get some discussion going.

I'm working on a 750 Norton motor. It is a Combat Commando.
I have to replace the push rods (it's a long story)
I have already ordered some chrome moly replacements
Stock ones:
Solid aluminum 0.375" shaft with hardened steel ends
Chrome moly ones:
4130 Chrome moly 0.3125" dia 0.083" wall thickness (These aren't the ones I bought, I think these are Maney)

Someone I was talking to said that Steve Maney (think Pops Yoshimura for Brit bikes) recommends halving the valve lash when using his steel pushrods. Stock is 0.008" in and 0.010" Ex (COLD) and Maney recommends 0.005 and 0.006.
It never occurred to me that changing the pushrod material would be a reason to change the valve clearances.

What do you think, and WHY?
 
bitzz said:
I'm bored. Let's see if I can get some discussion going.

I'm working on a 750 Norton motor. It is a Combat Commando.
I have to replace the push rods (it's a long story)
I have already ordered some chrome moly replacements
Stock ones:
Solid aluminum 0.375" shaft with hardened steel ends
Chrome moly ones:
4130 Chrome moly 0.3125" dia 0.083" wall thickness (These aren't the ones I bought, I think these are Maney)

Someone I was talking to said that Steve Maney (think Pops Yoshimura for Brit bikes) recommends halving the valve lash when using his steel pushrods. Stock is 0.008" in and 0.010" Ex (COLD) and Maney recommends 0.005 and 0.006.
It never occurred to me that changing the pushrod material would be a reason to change the valve clearances.

What do you think, and WHY?
Click to expand...
Different materials lengthen at very different rates. Temperature stability.
 
Thermal expansion ... specifically, differential thermal expansion between whatever material the pushrod is, and whatever material the crankcase and engine block and cylinder head are made of (probably aluminum).

We can only estimate what the operating temperatures are, but let's make an assumption that since the pushrod is completely surrounded by fixed parts of the engine and with oil splashing around, the pushrod is the same temperature as the engine.

If that's the case, then as the engine heats up, the pushrod will expand together with the engine, thus little or no difference to the clearance if they're the same material.

If you replace the pushrod with a steel one, the engine block will expand faster than the steel pushrod grows in length, thus opening up the clearance, so it has to start out tighter, and from that point of view, their recommendation makes sense.

Aluminum expansion coefficient 24 x 10-6 / C
Steel expansion coefficient 13 x 10-6 / C

difference between the two is 11 x 10-6 / C

Plausibility check, if operating temperature is 100 C above ambient and the pushrod is (guess) 200mm long then this would give a differential length change of 0.22 mm = about 0.009".
 
Be really interesting to see the performance gain. Tighter clearances, valve overlap?
 
Another source gives aluminum 21 - 24 x 10-6 / C, and a number of alloy steels 14.4 - 17 x 10-6 C as opposed to just the generic "steel", so it's possible that the above calculation overestimates the length change a bit depending on what alloys are actually used. Coefficients of Linear Thermal Expansion
 
FWIW the steel pushrods will weigh about 50% more due to the higher density. The aluminum ones will also be stiffer even though aluminum has one-third the elastic modulus ... because of the bigger outside diameter.
 
If it has less inertia in theory they can move faster better.
;) tiny bit less likely to cause that nasty valve float problem.
 
On Chrysler slant sixes (before they switched to hydraulic lifters) you were supposed to adjust the valve clearances with the engine warmed up and running.

 
Sorry.
Chrome moly pushrods are hollow 5/16" 0.083 wall thickness. They weigh less than stock.

and being a push rod motor all the htermal expansion I am really worried about is the cases, block and head.
The difference in length of the pushrod is going to be negligible, from a 3/8 solid aluminum to a 5/16 hollow 4130... I think.
 
On your Dodge six that you set the valve lash at operating temp, what's the lash?
I am well familiar with solid lifters in cars.
IIRC on a 383 Dodge 0.012" cold gets about 0.035" hot.
 
Aluminum 0.375" (9.525mm) solid round, x-sect area 0.71 cm2, Density 2.6 g/cm3, weighs 1.85 grams per cm of length
Steel 0.3125" (7.9375mm) with a 0.1465" (3.72mm) hole up the middle, x-sect area (0.495 - 0.109) = 0.39 cm2, Density 7.8 g/cm3, weighs 3.83 grams per cm of length

So the aluminum solid will weigh less than the steel hollow of those dimensions for the same length.
The end-fittings would obviously be on top of that.
 
Brian P said:
On Chrysler slant sixes (before they switched to hydraulic lifters) you were supposed to adjust the valve clearances with the engine warmed up and running.

Click to expand...
I don't mean to derail the thread, but at first glance this seems.... very unnecessary. You posted some perfectly good math already, was there a reason that was not sufficient for Chrysler to determine a useful *cold* valve clearance spec?
 
The reason I bought chrome moly push rods is Manton will make 5/16 4130 custom pushrod for $11.50, I called them Friday morning, they shipped them this morning.
I personally believe there is NO performance advantage available in Norton pushrods. Stock ones are fine.
I just needed custom length, and i bought the cheapest ones I knew of.
I just can't figure out WHY you would close up the clearances if you use steel pushrods.
 
It's an awesome bike although I did like the interstate fuel tank better ;) closer clearance just means the rod is not going to grow as much for whatever reason, like maybe it contains no copper or maybe it's even sodium cooled, that would be very cool.
 
My cousin has had two Norton Commandos.His 72 was stolen 6 months after he bought it.The insurance replacement 72 he still has with i think about 20k miles on it.Been in storage for decades.I remember him telling me that he loved it,but it was a pia bolting it back together after every ride.Lol.
 
Oh for sure they were horrible machines to keep running lol but omg did they ever feel fast.
 
Engines don't get hot enough to expand steel pushrods. Aluminum expands at much lower temps so you need more clearance with Aluminum .
 
That's totally true, if it got too hot the oil would burn ;) but they all expand, that's why they need the valve clearance to begin with.
 
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