Re shimming heads and hydraulic lifters

[J.A.C mech]

Done a search around and have not found anything to conclusive whaich answeres my questions.

basically what i would like clearing up if anyone could help is why my head would need reshimming with a new cam.

I have a 1.9 td pug engine with a std cam,

i rang piper cams to obtain a upgraded one, they offer a:

Blank cam for around £220 + vat or
Regrind for around £100 + vat

so i understand that using a reground cam will mean the cam lobes have been machined smaller meaning you have to make this up so the valves are pused to there corect distance into the cylinder, by reshimming the head with thicker shims to make this up.

However i was under the impression that using a blank cam that the cam is made to the same spec as the origional but obviously with different shaped lobes which i would imagine open the valves for longer.

but i have been advised that it will still need reshimming, how does this come about?

Aslo some one said that if you have hydraulic lifters if you 'cam' the car theres no need to shim the valves, how does that come about?

any help and knowladge would be greatly recieved,

many thanks Joe

[GreenV8S]

The clearances in the valve train have to be right to within thousands of an inch.

If you have hydraulic lifters these are self adjusting, so the valve train has to be assembled with everything positioned 'about right' and the hydraulic lifters will sort everything out for you and adapt to changes as things expand and wear. Even hydraulic lifters have to be set up correctly though, they only have a small range of adjustment and you need to ensure that they're fitted with the right preload so they stay within their working range.

If you don't have hydraulic lifters or any other form of self-adjusting mechanism then you will need to set the valve train clearances manually and reset them periodically to accommodate wear and tear. In this case even if you just replace one cam with another cam of the same space you will need to reset the clearances afterwards because the parts are never *exactly* the same. If you're changing them for a different spec of cam then you'll definitely need to set the clearances and may in some cases even end up having to change other components changed/machined to get the clearances right - there's only a finite range of adjustment that can be achieved with shims.

Edited by GreenV8S on Saturday 19th January 18:47

[J.A.C mech]

thanks for that chief,

so really either cam will need shimming and even custom shims for either too, so i would be just as well getting th re-ground cams!

thanks again for explaing