Its infinitely easier to use a leather strap (leather also lets you do the whole 42-45 mularky and webbing is pretty much late 44 onwards)
we did all ours by drilling the helmet band on the liner and attaching the leather straps to that.
Otherwise you've got to drill through armoured steel and it simply destroys drill bits as its *designed* not to be punctured easily.
The only easy way of doing it is to heat the helmet shell up until its red hot and punch it through.
But then you wreck an RAC helmet which is collectable in its own right now. Using the liner method you only damage the liner and then it's barely noticable. I can change my 'para' helmet back into a 1944 dated RAC one in less than a minute.
My honest advice, dont bother trying to convert it to take web straps.
Give Sgt Heide (pete) on the forums a pm, he was making some excellent fibreglass repro para lids that would be much easier to convert and save you wrecking a collectors piece.. scrimmed up these are undetectable from a real para lid.
Other wise be prepared to spend ages and get through a lot of drill bits trying to put holes in a battle helmet.
Scaleyback managed to 'hole' a few but he was wokring in afoundry at the time and had the facilities to heat and punch the metal properly.
If you *really* want a good late pattern airborne helmet get a belgian one, the web straps are slightly greener but other wise its the same.
I just dont think it's worth the hassle trying to make one out of an RAC shell tbh, for the time and effort it will take you may as well buy one predone from steve kiddle at pegasus.
"I think we are in rats' alley - Where the dead men lost their bones."
Well i mean as a piece of miliaria a few holes will wreck it's value.
Hence its better to get a repro liner and attach the chinstrap to that.
Depends on what you paid for your RAC lid. Five years ago you could get them for 15 quid (my 44 dated one was only 20) but now they easily fetch 50 to 60 quid as tankie lids.
A steve kiddle conversion with all the work done for you is about 100 quid so it just makes more sense.
Basically putting holes in the shell is loads of work and will wreck the vlaue of your lid and if you just want it to look like a para helmet for airsofting/re-enacting and dont actually need it to offer any jump, bump or ballistic protection it makes loads more sense to attach a leather chin strap to the liner.
If you pierce the lid you're going to have to fill the old bale hole anyway or have two sets of holes which will look as odd, granted if you attach the chin strap to the liner the bolts/rivets on the shell are in slightly the wrong place but its not that noticable and near impossible to tell once scrimmed.
If you've got access to decent machine tools/a workshop and dont care about the helmet then go for it but seriously we looked into this a lot for the whole group and the best option was putting the chinstrap on the liner.
To attach the leather strap the liner:
Take the liner out and drill a hole in the cente rear and one each side at the points you want them at, then just bolt the chin strap on. Obviously you need to cross over the two parts at the back to join both parts together with one bolt.
If you're clever you can leave the exsiting elasticade RAC chin strap on and just put it above the liner between liner and shell. That way of you unbolt the leather chinstrap you've only got three 5mm holes in your liner that are invisible unless you're looking for them rather than three 1cm holes punched in the shell.
Somewhere i think diesel monkeys got a load of stage by stage pics of how to do it.
I'll see if i've got any pics to hand.
"I think we are in rats' alley - Where the dead men lost their bones."
Cant find any detailed ones but if you look in this pic I'm pretty sure *all* the helmets in it have been done using the liner method. Mine has for sure (i'm the Sgt)
edit: actually Yith with the Sten is wearing a pegasus repro.
Once i'm back from the Cold War battle this weekend and assuming no ones beat me to it i'll take some pics of my liner with the attached chin strap for you.
"I think we are in rats' alley - Where the dead men lost their bones."
We all bought the SOF ones and they are pretty good. Mine is ever so slightly starting to decompress (as in the layers pressed together of 'leather' are startinf to seperate) which is making it a little harder to fasten up quickly but it has lasted about six years of being used for s few months each re-enactment season, so i think they are worth it.
Like yourself I bought my RAC lid with the intention of converting it 'properly' and then saw how collectable war dated RAC lids are becoming in their own right and thought twice about it (mainly because everyone in the last ten years has been banging holes in them to make para conversions.. a bit like 49 pattern kit.. once you couldnt give it away now its hard to find in sensible sizes).
I dont 'convert' anything thats original from any period any more. Given there are decent repros for most wwii stuff you're just sabotaging militaria converting it 
I think the conversion we did cost about 22 quid (20 quid for the strap and 2 quid in bolts and washers) and took all of five minutes to do. I've been looking through my photo files but cant find the ones of the conversion. I'm packing for 'the mole' tonight but if i dont get chance to take pics of the positions to put the holes in tonight i'll sort it out sunday night for you.
"I think we are in rats' alley - Where the dead men lost their bones."
Cant find any detailed ones but if you look in this pic I'm pretty sure *all* the helmets in it have been done using the liner method. Mine has for sure (i'm the Sgt)
Nah, mines a Pegasus too, but I could only tell it wasn't my old RAC conversion because of the colour of the edge of the strap.
