pre load on rockers

[sparky]

Having had my heads skimmed on my V8, can anybody tell me whether I need to adjust the push rod pre load, or do the hydraulic followers adjust automatically?

[Bowie69]

You may need to yes, but you may also be within tolerance anyways.... check the preload once you have the heads and rockers torqued down, you will probably find the pre-load WAAAAY too much, especially as you have skimmed the heads.

For example when I built mine (4.0l) recently I had 15 thou skimmed off the heads, nothing off the block, when checking the pre-load it was somewhere in the ballpark of 150 thou(!). I used shims to correct to between 40-60 thou and all good.

If the pre-load is too high you run the risk of the followers pumping up prematurely and the valves never shutting properly -you can probably guess the rest...

[sparky]

You may need to yes, but you may also be within tolerance anyways.... check the preload once you have the heads and rockers torqued down, you will probably find the pre-load WAAAAY too much, especially as you have skimmed the heads. 

For example when I built mine (4.0l) recently I had 15 thou skimmed off the heads, nothing off the block, when checking the pre-load it was somewhere in the ballpark of 150 thou(!). I used shims to correct to between 40-60 thou and all good.

If the pre-load is too high you run the risk of the followers pumping up prematurely and the valves never shutting properly -you can probably guess the rest...

Cheers mate this does confirm what I thought, dont think I dumb but how do I check the clearance now and then how do II work out what shims to put under the rocker, RPI talk about rocker ratios and stuff, any advice?

[Bowie69]

Ignore RPI, and follow the much better advice of BBC, in a thread on this very forum:*CLICKY*

You can order a shim pack from a number of places, one example being here:*CLICKY*

Take your time, measure and re-measure making notes as you go, and you'll get there, it makes a LOT more sense once you have everything bolted together

Rocker ratio is 1:1.6 so if you need to remove say 30 thou of pre-load on the lifters on average, then you need an 18 thou shim (or thereabouts) to get within tolerances. Do not use different shim thicknesses on the same rocker shaft, you will bend/break something(!), instead work it out to be 'on average' within tolerance.

You may of course find everything is fine without shimming, but I somehow expect not

[sparky]

Ignore RPI, and follow the much better advice of BBC, in a thread on this very forum:*CLICKY*

You can order a shim pack from a number of places, one example being here:*CLICKY*

Take your time, measure and re-measure making notes as you go, and you'll get there, it makes a LOT more sense once you have everything bolted together

Rocker ratio is 1:1.6 so if you need to remove say 30 thou of pre-load on the lifters on average, then you need an 18 thou shim (or thereabouts) to get within tolerances. Do not use different shim thicknesses on the same rocker shaft, you will bend/break something(!), instead work it out to be 'on average' within tolerance.

You may of course find everything is fine without shimming, but I somehow expect not

Well chuft with your advice so far thanks a lot, I am getting the idea. Just one thing can you clarify, I understand using the welding rod technique under the push rod stuff, but when you say 'then meausure' what exactly am I measuring, I presume a gap somewhere, but can you tell me exactly where, cheers mate you've been a great help to me.

[Bowie69]

Use the tip in BBC's thread and loosen the rocker bolts until the pre-load is within tolerance, and then stick a set of feelers under the rocker post - this will give you an idea of what you need shim-wise. You can measure the actual pre-load using a dial gauge, but in reality the above method is more than accurate enough, just check them all again once torqued down that they are within spec an you are done