gerg

Pushrod Length Checking Tool

12 posts in this topic

After getting your head/deck/valves/seats machined, the required pushrod length will be shorter. As there is some very complex maths involved, best way to find the new length is to assemble it all, including torquing the head down, and physically check it with this tool.

 

Sure you can buy them, but I always say "why buy something that you can make for nothing?"

 

Starting with a stock Clevo pushrod, I cut it about 2 inches from the end. Then i cut another inch off the longer bit so that there's an inch piece missing from the total length.

 

The stock 5/16" pushrod has an ID of about 3/16", so it's roughly a 0.060" wall. In metric that would be 8mm, 5mm x 1.5mm respectively.

 

The 5mm ID is perfect to tap an M6 thread into, which i did. I then cut the head off a 6 x 50mm bolt and installed 2 nuts.

 

I then drilled out 2 M8 nuts with an 8mm drill and slipped them over each cut end of the pushrod, and tacked them in place with a very light setting on the mig (no distortion, etc).

 

Wind each end of the bolt into the threaded pushrod tubes and tighten the locknut on one end. Make your adjustment on the other end.

 

Pics below:f36653ebe30e8be7f78f4c93e367f2f0.jpg8d09d16c7ccc4832c91e58f012f534da.jpg44af79cbebc39a2ef82234d950036681.jpg

 

Sent from my CPH1607 using Tapatalk

 

 

deankdx, Banno, 2redrovers and 6 others like this

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Just not sure of how much has been taken off the heads already, and just had 25 thou off now. Block, who knows... 5, 10 thou? Valves and seats, same (puts valve tip higher). Believe me, I'd rather use stock length as they're easier to come by but i dont want pump-up issues especially since i plan on going to roller rockers. The current stock rockers are a tad worn so that will increase the height of the pushrod cup slightly. If the whole thing works out without needing shorter ones, then happy days.

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bear351c and CHESTNUTXE like this

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nice work on the tool,just remember depending on what brand of roller rocker yella terra should be fine but some vary in the cup i found this out on my crossy as i needed longer pushrods even tho heaps had been shaved off block and heads,b4 ordering any pushrods do your assembly and you might be surprized at what the size turns out to be:P

gerg likes this

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There are a couple of ways you could check it. You could:

A) bolt the rocker on and adjust the checker to the required preload (fiddly, needs a dial gauge).

B) using a 12" vernier, set it at a known available pushrod length, lock it and adjust the checker to suit. Install the checker and see how many turns of the fulcrum bolt it needs to go from zero lash to tightening on the pedestal.

I chose option B.

Some things to think about with pushrod lengths:

Too long: higher preload puts greater force on the cam base circle. Also gives the lifter more travel before "topping out" against the c-clip, holding the valve open further when pump-up occurs. Not much of a problem with stronger valvesprings.

Too short: can cause lifter plunger to pump up against c-clip and pop it off, causing a world of pain.

Correct preload varies depending on thread pitch of the rocker bolt, type of lifter, anticipated wear of valvetrain components, etc.

Most lifter manufacturers specify 0.060-0.080" preload. On a stock 5/16" UNC-18 pedestal bolt, that's 1 to 1-1/4 turns. If it was on a stud-mounted 7/16 UNF-24, it would be closer to 1-3/4 turns.

Total lifter travel is normally accepted as 0.120" but you don't want to be in the upper or lower reaches for reasons already stated.

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bear351c and CHESTNUTXE like this

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Yeah I'm a bit concerned about that. I think i might go with the Precision rollers. Best priced USA made units and bolt-on too.

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CHESTNUTXE likes this

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i have made that mistake as well thinking aarrr there hyd there is no adjustment but there is ,getting that preload correct is a good skill to learn,i usualy leave the manifold off and get it on tdc for each cyl and watch the plunger go down a tad and count the turn of the bolt per rocker,sometimes just 1 can be way out as the valve has been sunk a bit more than the others.or many use the other method ehaust closing inlet opening method ,both work well,but i have had better off the bat success with doing them on tdc method.

gerg likes this

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slightly off topic but one of the issues my 351 had back in the day with everything surfaced was the inlet manifold not sitting on neat.. needed silastic to seal 

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slightly off topic but one of the issues my 351 had back in the day with everything surfaced was the inlet manifold not sitting on neat.. needed silastic to seal 
Sure it wasn't angle-milled? Everything should still line up except your port roofs can be out of whack to the manifold runners. Of course there could have been a shitload taken off the block deck, meaning your end rails sat too high. Dunno about that one, got me stuffed.
Could you go for 1.6 ratio rollers, instead of 1.73.? Obviously this would alter valve lift..
That would make a miniscule difference to the preload amount and i doubt you can get pedestal mount rockers in 1.6 anyway, valve lift and 0.050" duration would suffer too.

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This arvo, all my gaskets and shit arrived from Precision. So I got my heads torqued down and compared a stocker 8.408 pushrod with the checker set at 8.383.

Tightening a rocker down on a pumped-up lifter just opens the valve. I had to remove the lifters and bleed them down in a vice.

Once bled out, it's quite hard to determine when the preload starts because the lifter spring is quite soft. You need to grab the pushrod and rotate gently while tightening the rocker bolt. As soon as you feel resistance, that's where you start counting how many turns on your rocker bolt you need to tighten down.

Checked on cyls 1 & 3, inlet and exhaust, stocker went 1-3/8 turns (0.075") of preload. Shorter one went 7/8 turn (0.048"). So longer one had 1/2 a turn or 0.027" more preload.

So allowing for the fact that the rockers are really worn, lessening the effect of too-long pushrods, I think I'll be ordering the shorter ones.

Sent from my CPH1607 using Tapatalk

bear351c and deankdx like this

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