11 replies to this topic

[racingbob]

Have built up some early pressed steel rockers with 1.4 bushes in problem is to put them on i suppose I need to take off the head and replace head gasket.my mates car was ok his gasket didn't leak after putting on his 1.5 what you reckon gives another 35 though lift it's has mg metro cam

[whistler]

Why would you need to replace the head gasket? I've changed rockers during a Rally and never had a problem.

[carbon]

+1. You should not need to change head gasket for rocker assembly change. I have also done this a couple of times with no gasket issues.

 

Drain coolant below level of head gasket, loosen off all head studs in sequence, then gently nip up 5 front studs and the 10th stud and bolt if you have these fitted. Remove/replace rocker assembly and then retorque head as per usual.

 

You will be very lucky to get as much as 1.4 ratio using offset bushes. You will also need to change or do work on the rocker pillars to allow for the slight difference in spacing from shaft to rocker tip when using the offset bushes.

[racingbob]

is that drilling the pillars necessary if using large pad pressed steel rockers ?

[hhhh]

If you don't relocate the rocker pillars, you will only gain the ratio increase on the pushrod side of the rocker, because that's fixed by the distance from ball end to rocker shaft. If the distance from the valve tip to the rocker shaft isn't increased, you'll lose roughly half the lift increase. In fact, you could get a slight increase possibly from standard equipment rockers by drilling the pillars and moving the shaft away from the valves, assuming the pad doesn't leave the valve tip at extremes.

[racingbob]

If you don't relocate the rocker pillars, you will only gain the ratio increase on the pushrod side of the rocker, because that's fixed by the distance from ball end to rocker shaft. If the distance from the valve tip to the rocker shaft isn't increased, you'll lose roughly half the lift increase. In fact, you could get a slight increase possibly from standard equipment rockers by drilling the pillars and moving the shaft away from the valves, assuming the pad doesn't leave the valve tip at extremes.

ok ta thought i read this somewhere

[hhhh]

by the way, if I remember right, I had to drill the rocker pillar holes larger than what Vizard recommended in order to get the movement required. And I've also changed them many times without lifting the head. I didn't even bother to drain the coolant which was probably a bad idea in hindsight.

[carbon]

is that drilling the pillars necessary if using large pad pressed steel rockers ?

Large pad pressed rockers are from pre-A+ 1275 motors. The ides of the wider pad is to allow for the sideways offset of the valve guides from the pushrods. They don't line up, and this sideways offset is over 3mm for centre exhaust valves on a 940 head (if you do line up the 1275 rocker with valve tips when using very high lift rockers do also check the pushrod to head clearance... it can be very tight).

 

Drilling the rocker pillars or using rocker pillars with suitable shaft offset is all about getting the rocker tip as close to the valve centreline as possible. And as hhhh says avoiding the rocker pad hitting the edge of the valve which would cause rapid wear.

[racingbob]

anyone know what size drill to use for pillars, i suppose best take to machine shop

[hhhh]

You don't need a machine shop because the holes are already piloted so it can't go too far wrong. With offset bushes, determine the offset, e.g.: 0.060" off center. This will be the radius increase required in your pillar holes. Double this to obtain your hole diameter increase, 0.120" for this example. Add this to your existing hole size, e.g.: 0.375" for the big holes to obtain 0.495". You'll want to incrementally drill the holes using 13/64", 7/16", 15/64" and finally 1/2" to get your final size which will give you an extra 0.005" for tolerance issues. Use the same method for the small holes which are 5/16" nominal. Don't forget to confirm that your pillar oil gallery still aligns. If not, countersink as required in the pillar enough so that the gallery in the head is not obscured at all.

Edited by hhhh, 28 April 2017 - 02:50 PM.

[Ethel]

It's moving the rocker shaft that really alters the ratio. Even on the pushrod side the cam follower still goes up and down in the same place under the rocker arm regardless of how the pushrod connects the two.

Whether the head gasket keeps its seal will depend on how flat the uncompressed head is, but a valve spring can lever the head up off its pushrod if it's still compressed as the head's unbolted.

[hhhh]

It's moving the rocker shaft that really alters the ratio. Even on the pushrod side the cam follower still goes up and down in the same place under the rocker arm regardless of how the pushrod connects the two.

Whether the head gasket keeps its seal will depend on how flat the uncompressed head is, but a valve spring can lever the head up off its pushrod if it's still compressed as the head's unbolted.

You are right on the valve side of the rocker, but while you are technically correct that moving the rocker shaft alters the effective ratio on the pushrod side, this effect is due solely to the misalignment of the pushrod with the centerline of the follower's motion and is so negligible as to be unmeasurable at the valve within measurement error.

 

Before untorquing the head, all the valve adjusters should of course be slackened as far as possible. This should mitigate the force of the valve springs. The head bolts don't need to be slackened so far that the head can move. Ideally, maintaining about 5 ft-lbs, i.e., keeping them snug, will work well.