Pushrod Length for Shaved Heads

-
71GSSDemon

71GSSDemon

FABO Gold Member
FABO Gold Member
Joined
Jul 27, 2018
Messages
7,692
Reaction score
17,996
Location
WI
Does anyone know how much can be shaved off the cylinder head's flame face before needing needing new (shorter) pushrods? Hydraulic flat tappet cam and stock rocker arms. The Mopar Direct Connection bible calls out to mill 0.060" off the heads to bring compression up to ~9.8:1 on 1970's low compression big blocks. I know there is a formula to also remove material off the intake side of the heads to keep that correct, but nothing is mentioned for the pushrods. Any help would be greatly appreciated.
 
Last edited:
66fs said:
Easiest way is to shim the rocker arm shafts the same amount. Or you can get a pushrod checker from Trend or smith brothers and get the right pushrods.


hughesengines.com

ROCKER STAND SHIM .040" :: HUG-1709

SET OF 10 Used to adjust rocker shaft height for achieving proper valve train geometry and pushrod length.
hughesengines.com hughesengines.com
Click to expand...
I cannot agree with that. The best way is to actually measure for correct length pushrods. The reason for that, especially on the Mopar small block is the drastic pushrod angle. You cannot just say "shorten the pushrod the same amount you mill the head". That doesn't work. You have to measure to get it right and to the geometry and rocker sweep correct. But now, we're getting into rocker shaft relocation and that's probably better saved for another thread.
 
RustyRatRod said:
I cannot agree with that. The best way is to actually measure for correct length pushrods. The reason for that, especially on the Mopar small block is the drastic pushrod angle. You cannot just say "shorten the pushrod the same amount you mill the head". That doesn't work. You have to measure to get it right and to the geometry and rocker sweep correct. But now, we're getting into rocker shaft relocation and that's probably better saved for another thread.
Click to expand...

I won't argue about what is best, but I have run shims on .040 milled J heads and 340 hydraulic rocker arms and that engine ran better than it had a right to. Never had a problem.
 
66fs said:
I won't argue about what is best, but I have run shims on .040 milled J heads and 340 hydraulic rocker arms and that engine ran better than it had a right to. Never had a problem.
Click to expand...
There's a lot of room for error with most hydraulic lifters. The plunger travel on most is around .250" and more in some cases, so that's a lot to play with.
 
I am specifically discussing Big Blocks. All info is appreciated though too!
 
71GSSDemon said:
I am specifically discussing Big Blocks. All info is appreciated though too!
Click to expand...
They also have an exaggerated pushrod angle. Not as much as a small block, but still enough to certainly figure into the equation. I'm sure it'll run if you just slap some shims in it.....or not......but if you're going to do it, why not measure and get it right?
 
RustyRatRod said:
They also have an exaggerated pushrod angle. Not as much as a small block, but still enough to certainly figure into the equation. I'm sure it'll run if you just slap some shims in it.....or not......but if you're going to do it, why not measure and get it right?
Click to expand...
Thanks Rob! Just polling, I agree on checking the length. Curious about experiences while the heads are out to be machined.
 
I never milled my big block heads that much, so I just bolted the stock rockers, shaft and pushrods on and ran them. Today, I get a pushrod checking tool and measure optimum length and buy custom pushrods. But I do know the shims work, pretty good. Perfect takes time and money, I finally have both.
 
Too bad I can't get the increase in compression by just using the 516 closed chamber heads vs the 452s. I would just do the valves and run those.
 
RustyRatRod said:
There's a lot of room for error with most hydraulic lifters. The plunger travel on most is around .250" and more in some cases, so that's a lot to play with.
Click to expand...
Yup.. that and add in the slop from the stock stamped rocker arms and you really have a LOT of room.
 
The factory & most aftermarket hyd lifters have 0.150" [ sometimes a little more ] range of adjustment, ie with the p'rod cup touching the cçlip, the cup can be pushed down 0.150" or until it bottoms out. This amount of travel is to allow for production tolerances, head milling etc. With the lifter on the base circle of the cam, anywhere in the 0.150" range is ok.
So before milling the heads, assemble & check out how far down the cup is from the c clip. If it is, say, 0.040", then you can easily remove 0.060" from the heads, & no shimming of the rocker shafts & no new p'rods are reqd.
To determine how much internal travel your lifters have, compress one in a vice & measure. If the lifter has oil in it, compress slowly to expel the oil.
 
Bewy said:
The factory & most aftermarket hyd lifters have 0.150" [ sometimes a little more ] range of adjustment, ie with the p'rod cup touching the cçlip, the cup can be pushed down 0.150" or until it bottoms out. This amount of travel is to allow for production tolerances, head milling etc. With the lifter on the base circle of the cam, anywhere in the 0.150" range is ok.
So before milling the heads, assemble & check out how far down the cup is from the c clip. If it is, say, 0.040", then you can easily remove 0.060" from the heads, & no shimming of the rocker shafts & no new p'rods are reqd.
To determine how much internal travel your lifters have, compress one in a vice & measure. If the lifter has oil in it, compress slowly to expel the oil.
Click to expand...
I can see you've actually never measured the full travel of a hydraulic lifter plunger.
 
71GSSDemon said:
Does anyone know how much can be shaved off the cylinder head's flame face before needing needing new (shorter) pushrods? Hydraulic flat tappet cam and stock rocker arms. The Mopar Direct Connection bible calls out to mill 0.060" off the heads to bring compression up to ~9.8:1 on 1970's low compression big blocks. I know there is a formula to also remove material off the intake side of the heads to keep that correct, but nothing is mentioned for the pushrods. Any help would be greatly appreciated.
Click to expand...
Instead of going to extreme lengths by drastically shaving your heads you might look at some of the modern camshaft profiles out there that compensate for low compression.
 
dart4forte said:
Instead of going to extreme lengths by drastically shaving your heads you might look at some of the modern camshaft profiles out there that compensate for low compression.
Click to expand...
Yup, some of those type camshafts are quite surprising.
 
dart4forte said:
Instead of going to extreme lengths by drastically shaving your heads you might look at some of the modern camshaft profiles out there that compensate for low compression.
Click to expand...
This engine has a brand new cam in it. I stole the heads for another motor and wish to replace them with something comparable. Just looking for experiences. Thank you
 
RustyRatRod said:
Why can't you?
Click to expand...
Ok Rob you have my attention. So, on a standard big block, when changing from, lets say 452 head and using a 516 head about how much compression would it increase? Other brands going from open chamber to closed chamber it is a drastic increase in compression. It appears through reading, the change alone nets maybe a 1/2 point or less, then the ports are not as good also, not to mention smaller valves and no hardened seats. So, decking the 516 heads, changing the valves, installing seats, etc would no longer be cost effective, or am I misinformed?
 
So you are working with like a '74 440 with cast pistons about -.110in the hole at TDC ? I would believe the simplest, best bang for the buck, and possibly the cheapest would be to buy the alum Edie heads.

Reasons I say this, is that starting compression is probably less than you think, closer 7.5 than 8.5 with stock open chamber heads, and good head work is expensive. The deck/intake cut ratio is about 1: 1.2; you cut more off the intake surfaces. It takes four mill set ups and time is money. After not having some heads milled for over 20 years, the last set I had cut about 2 years ago I about choked on the price quoted. We flipped a coin for $50 of the charge, I won, and paid $250 for the milling. Makes buying valves seem like a better deal, especially if improved with at least the 1.81" bigger exhaust.

If you do have the low comp pistons, my second choice after the new alum heads, would be use the old closed chamber heads and buy new bigger valves, you probably need new valves anyway no matter which heads you have rebuilt.
 
Outsder said:
So you are working with like a '74 440 with cast pistons about -.110in the hole at TDC ? I would believe the simplest, best bang for the buck, and possibly the cheapest would be to buy the alum Edie heads.

Reasons I say this, is that starting compression is probably less than you think, closer 7.5 than 8.5 with stock open chamber heads, and good head work is expensive. The deck/intake cut ratio is about 1: 1.2; you cut more off the intake surfaces. It takes four mill set ups and time is money. After not having some heads milled for over 20 years, the last set I had cut about 2 years ago I about choked on the price quoted. We flipped a coin for $50 of the charge, I won, and paid $250 for the milling. Makes buying valves seem like a better deal, especially if improved with at least the 1.81" bigger exhaust.

If you do have the low comp pistons, my second choice after the new alum heads, would be use the old closed chamber heads and buy new bigger valves, you probably need new valves anyway no matter which heads you have rebuilt.
Click to expand...
The question from above remains though. What increase of compression could be expected by changing from open chamber heads to closed chamber? Also, I agree the aluminum aftermarket heads would be ideal, or tear the short block apart for better pistons. If using the 516s, new valves, surface, hardened seats, etc it doesn't seem like a cost effective way to go nowadays. Milling the heads was quoted at $150. Yes 0.0123" taken off the intake side for every 0.010" taken at the flame face. The heads are 452, valves are good and they are factory with hardened seats. Seems like the way to go in this situation.
 
Last edited:
Could be pretty similar, closed chamber Vs. milling the open chamber 0.060", though you would have to CC both sets of heads. Usually the stock chambers are bigger than the blueprint specs by quite a lot. The alum heads have pretty small chambers and the advertised numbers pretty close.
 
Outsder said:
Could be pretty similar, closed chamber Vs. milling the open chamber 0.060", though you would have to CC both sets of heads. Usually the stock chambers are bigger than the blueprint specs by quite a lot. The alum heads have pretty small chambers and the advertised numbers pretty close.
Click to expand...
I just looked up the E street heads. Although these would be nice, I would have less than half the investment even adding the 2.14" valves. Budget is always the concern.
 
If you have some good old closed chamber heads and a good relationship with the shop, you can make it work well. Have them Mag the heads, put in bronze guides and rough in for the bigger valves. Take the heads back home, carefully blend the pockets yourself and port match if you care to. Then take back to the shop for the finish valve job. You won't notice the "old style" port issue if done right.
 
71GSSDemon said:
The question from above remains though. What increase of compression could be expected by changing from open chamber heads to closed chamber? Also, I agree the aluminum aftermarket heads would be ideal, or tear the short block apart for better pistons. If using the 516s, new valves, surface, hardened seats, etc it doesn't seem like a cost effective way to go nowadays. Milling the heads was quoted at $150. Yes 0.0123" taken off the intake side for every 0.010" taken at the flame face. The heads are 452, valves are good and they are factory with hardened seats. Seems like the way to go in this situation.
Click to expand...

This old thread has lots of good info. Might be worth a read-through.
 
You must log in or register to reply here.
-

Back
Top