When changing the valve springs on a blacktop can you use the existing shims or do you have to change them for new... I’ll try my hand at most things but i think i’d find my engine building limits rather quickly trying to measure shims and gaps etc

You can use them if there in good serviceable condition and more importantly in tolerance.

You build the head up and measure all the gaps with a feeler gauge. You keep moving them and re measuring them until you get the best tolerances. If there out you have to buy them at what ever size you need.

You will save your self hours if you don’t mix them up when stripping it down.

Just fitting the st springs won’t change the shim tolerances. Lapping in the valves will.

Unless your going mad you only need them on the inlet to stop valve float.

The st has differant length springs on the inlet / exhaust. So don’t mix them up.
Also turn the cams before you fit the head as the the longer springs can coil bind on some cams.

[Edited on 12/6/19 by big_wasa]

It’s just going to be st springs, so if i keep the shims in/on the same port i should be ok?

Depends on ware and tare but yes the distance between the valve seat and cam lobe will remain the same.

Make a note of the shim thickness before you build the head up , possibly like a spread sheet , then measure the clearances then do the maths to arrive at the correct size shim . Lapping the valves in will reduce the clearances but only very slightly unless the seats and valves are refaced.

As mentioned you can measure the gaps using feeler blades (a set of feeler blades aren't expensive and are handy to have).

You can measure the shim thickness using digital vernier calipers. You can buy a set of calipers that measure millimeters to 2 decimal places for less than £20 from Halfords. Again a handy tool to have for measuring many things and they measure internal and external distances as well.

https://www.halfords.com /workshop-tools/tools/hand-tools/laser-digital-vernier-caliper-150mm?_br_psugg_q=vernier+caliper

https://www.screwfix.com/p/magnusson-digital-vernier-caliper/7093v

[url=https://www.screwfix.com/p/hilka-pro-craft-32-blade-feeler-gauge/4406r]https://www.screwfix.com/p/hilka-pro-craft-32-blade-feeler-gauge/4406r[/ur l]

[Edited on 13/6/19 by nick205]