Working out Pushrod length

There are lots of variables on this engine so I'm trying to work out how long my pushrods should be - If I try to explain how I'm doing it perhaps someone who knows could tell me if this is correct.
I've made up a replica pushrod with a threaded section in the middle so that it can be adjusted.
A six inch bar threaded into the rocker arm as an adjuster making it easier to see if the adjuster is inline with the valve stem.
I've measured the length of the valve stems protruding from the guides when closed. I then turned up some pins that locate in the top of the guides that imitate the valves at half valve lift.
Set the degree disc at TDC and then rotate the crank until the one of the cam lobes is at half cam lift and mark this on the degree disc.
Put the head on with the appropriate pin in the guide and the adjustable push rod.
With the crank then set at half cam lift adjust the 6" adjuster until there is zero clearance and then seat the square end of a 6" rule to where the valve springs would seat in the head and eye up this with the adjuster to see if the is inline with the rule and therefore the valve stem at half valve lift. If it doesn't align then adjust the 6" adjuster until it does and measure the gap with feeler gauges. Strip it back down and adjust the pushrod length and repeat. This is done individually for each pushrod. - what do you think?

My 500cc Triumph engine was made out of Triumph 650 parts but had a 63 mm stroke billet crank. The barrels and pushrod tubes were shortened by 12mm, so that is how much I shortened the pushrods. If the base circle of the cam is bigger, you need to allow for that.

If you have a standard cylinder head then stock pushrod length is good. The only reason to change is if the head has been milled such a the combat 750 head. You might explain more why you need adjustable pushrod length. Also there is very little clearance in the pushrod tunnel of the head, so no room for a tube adjuster diameter more than standard. I put .060” shorter than stock chrome-molly pushrods from Alloy-Tech in my combat engine. Unfortunately that company from Tuscola, Illinois no longer is in business.

Interested in this. I have heard two schools of thought on the optimum length.

- make the length such that mid valve lift the adjuster is aligned along the valve such that the sweep is evenly distributed between closed and full lift. Which is what you seem to be aiming for.

- or make the length such that the adjuster is aligned to the valve as close as possible at full lift. The argument being this is when the force is highest and you don't want it pushing across the valve.

The second does seem to be sensible. What is the collective opinion?

Your method seems very good. But is half lift really what you should aim for. I don't know and interested in people views.

Is the second option even physically possible?

acotrel said:

My 500cc Triumph engine was made out of Triumph 650 parts but had a 63 mm stroke billet crank. The barrels and pushrod tubes were shortened by 12mm, so that is how much I shortened the pushrods. If the base circle of the cam is bigger, you need to allow for that.

Click to expand...

What?

Nourish said:

There are lots of variables on this engine so I'm trying to work out how long my pushrods should be - If I try to explain how I'm doing it perhaps someone who knows could tell me if this is correct.
I've made up a replica pushrod with a threaded section in the middle so that it can be adjusted.
A six inch bar threaded into the rocker arm as an adjuster making it easier to see if the adjuster is inline with the valve stem.
I've measured the length of the valve stems protruding from the guides when closed. I then turned up some pins that locate in the top of the guides that imitate the valves at half valve lift.
Set the degree disc at TDC and then rotate the crank until the one of the cam lobes is at half cam lift and mark this on the degree disc.
Put the head on with the appropriate pin in the guide and the adjustable push rod.
With the crank then set at half cam lift adjust the 6" adjuster until there is zero clearance and then seat the square end of a 6" rule to where the valve springs would seat in the head and eye up this with the adjuster to see if the is inline with the rule and therefore the valve stem at half valve lift. If it doesn't align then adjust the 6" adjuster until it does and measure the gap with feeler gauges. Strip it back down and adjust the pushrod length and repeat. This is done individually for each pushrod. - what do you think?

Click to expand...

I wonder if I should of set the valve clearance as per cam fitting instructions?

There is some much on this site concerning Pushrod length - but nothing to ascertain the correct length in practice!

Nourish said:

There is some much on this site concerning Pushrod length - but nothing to ascertain the correct length in practice!

Click to expand...

The correct pushrod length is the one that keeps the motion of the path of the point of contact of the adjuster and valve stem centered on the valve stem. That is, it moves back and forth an equal distance from the center of the valve stem as it move towards and away from the rocker arm shaft.

The procedure you are describing should get you close enough, but I'd still check the actual path of the contact point to be sure. When making your adjustable pushrods, take care to keep the adjusters small enough in diameter to be able to still install them. You can get them made by Smith Brothers, see

https://www.pushrods.net/adjustable-pushrods

You can download the order form for them here https://www.pushrods.net/_files/ugd/eb4663_d7162da7fe5b45a28ef598a8512c92df.pdf

Or buy the adjusters from them and make your own. I don't see the individual components listed on their web site, but I've bought lots of push rod ends from them in the past with no problem. You might have to call them to make the order. I've attached a list of their individual components from their 2019 catalog here.



The procedure I usually use to check, is to paint the tips of the valve stems with steel blue (Dykem, metal marker pens, etc.), assemble the engine with the recommended valve clearances, turn it over a few times, and then look at the path of the contact point where the valve adjuster has worn away the blue. You can then shorten or lengthen the pushrods as needed.

The criteria that the line from rocker arm pivot center should be perpendicular to the axis of the valve when the valve is at half it's maximum lift is a pretty common recommendation. I have seen arguments for using other lift points (2/3 max lift is one), but the half lift point is what most tuning books recommend.

As long as the contact point doesn't get too close to the edge of the valve stem, it's probably fine for anything except a full race engine. Smokey Yunick's criteria was that the contact point should stay at least .020" away from the edge of the valve stem. Smith Brothers has a similar recommendation, shown here



Ken

Oops. You can ignore all the bits about making adjustable push rods. I missed in your post that you had already done so.

Ken

Thanks Ken. I was expecting to ascertain the correct length at just mid lift but yes checking the 'Skid Mark' as being the way to go. Thanks again