RNN14 Nissan Pulsar GTIR Online Workshop Manual | Covering B13 series
^^ that's a link to the service manual procedures for checking shims and calculating the clearance required etc. Hope that helps.
Also, some of the common size shims are very hard to get and/or no longer available.
This info stolen from another forum :
a quick and inexpensive fix on adjusting valve clearances:
my little bodge (and it does work, done it many a time) consists of nothing more than a set of feeler gauges!
check the clearances on your valves in the normal way, but when you work out shim size instead of buying shims to take up play, take the shim size and add a feeler gauge thickness to make correct clearance.
i.e if clearance should be 0.15mm and shim size is 0.20mm take a 0.05mm blade from the feeler gauge and cut a small piece off which will sit under the existing shim (make sure it is seated correctly) then place the shim on top!
thats it, simple and it works, and it will only cost you a set of feeler gauges to reshim your head, and no waiting around for weeks trying to get right size shims in!
it is best to use a joggler to cut out the shim from feeler blade as it will cut a perfect 6mm piece of shim which will fit snuggly inside the main shim.
also when measuring clearances from cam to rocker, use 2 feeler blades at a time (one each side) or the rocker may tip and give you an inaccurate reading