engine weak points/ placement

What consistently malfunctions on the gtir gear box?
Is it the gears, shafts, casing itself?

I would leave the stock liners if there all good and within spec, you don't need larger pistons and are best off sticking with 86mm
liners are stepped and a real mare to fit by any engineering company in the uk so you could be courting trouble by getting them fitted if not needed as they can lift especially if running high torque as the block can twist.
as mr B said you can safely go to 87mm pistons and some are using 87'5mm but I wouldn't as the thinner the liner the more heat is generated and transferred to the cooling jacket and as previously said its all about cooling and keeping all temps as low as possible if you want an engine that will last you any lengh of time.

at around 450hp you have a very fast car especially if its been lightened and I cant see the need to run more power on such a short wheel based car that's shaped like a house brick lol

stock gearbox is a lottery at that power but a par or quaife will suffice and handle it no problem providing you don't crunch gears.
ive personally never had a gearbox casing go on me (up to yet) but that will always be a risk if you launch the car 'which I rarely do'
you can brace it which helps keep the rear casing together but it wont stop the gearset from punching through the bellhousing end (don't spend hundreds on a brace as you can do the same thing for just a few quid which is just as effective.

I reckon a 10k budget might just get you there
3k gearbox
3k engine parts and engineering costs
1k gt3071r tubby
1.5k aftermarket management and mapping
500 injectors and z32 afm
1k clutch and other little bits youll need

then you need stuff like big brakes and suspension to enable you to use the power as the car will be useless if you cant stop it or chuck it round a bend lol

Well I wanted tk use the car at the strip mkre than id autox it, so it grenading at launch is an issue. What exactly cAuses the transmission to fail?
What caused the gears to crunch, what drives the shaft througj the casing, the point being, if I were able to find a place to custom fab trans parts out of any metal, where should I start.

Thanks again for all the help.

With standard helical gears main issue is shaft thrust against bearings. on aggressive launch & gear changes the on off load on the shafts causes them to thrust with great force against shaft end bearings & crack casing/punch right out. Other is is 2nd-3rd gear teeth failing.
PAR straight cut if built well should do you ok but any car gearbox used competitively & especially so for drag racing a 4wd is going have high risk of transmission repairs.
I would do the resin end brace even on straightcut gearset as it helps support end casing from too much flex/possible cracks from shafts being pushed apart by straightcut gears. Certainly worth every effort make most of a good gearset rather than "wish had done that" scenario.
Sings drag R here in Thailand used a PAR with strengthened case with some quite powerful VVL headed engines & took abuse really well, was looking at evo box conversion as so many gearsets available & cheap & even custom set made locally modelled on the PAR but simply wasn't needed or that affordable as a 1 or 2 off effort.
GTiR gets slated for its gearbox but in honesty it is most owners who are at fault rather than the box, standard box is super reliable at 350hp & many have got away with standard old box at 400 & up hp for quite some time & abuse.
I'm on 3rd gearbox build on evo so it not just GTiRs that bust boxes.
doubt your budget will allow for custom developing gears, just make top effort with the PAR build & use a dampened 6puk clutch to help it out.
A 450+ build should be pretty reliable with common parts available.

I could have sworn par were semi helical cut gears not straight cut dogset such as ppg
The teeth are slightly more straight cut than quaife but certainly not straight cut or am I missing something here?

As for stock boxes don't forget they are 20 + years old in many so is a lottery with stress qfractures in gear set unless of course you have a brand new stock gear set that's never been abused or crunched
How many gearbox builders X-ray the gears ect............hardly any I think you'll find

straight 1st to 4th dog or syncro + options. On about revision 3 on their GTiR set, no idea if semi helical was/is available .
ppg is nice but no way I could pay/warrant the price .

learn something new every day, johnny on here I believe has a par set and that's not straight cut
how much is the par straight cut set then out of curiosity?

about 3,600GBP with most extras thrown in, syncro option being little cheaper but not much with all extras.
if do semi helical they may be able offer mixed set so a 1st to 3rd straightcut & 4-5th semi helical, custom ratios are no extra cost.
Quite reasonable until add shippping-tax for uk & bearing/seals for decent rebuild.
Was/is a top option in Asia pacific due to cost & generally they make good kit for cost.
Main issue for uk is price not so good once add P&P/tax plus an early adopter of PAR had a right messy problem build, do believe first design from PAR was not best but still worked if built right.

As bob says Sure my par is not straight cut wines like hell especially second. Before anyone asks no its not standard and just fooked. Teeth are alot straighter than my quaife but as above learnt something new

I think you can still get semi helical but it not mentioned on parts sheet I have on the PAR.
They will do lots of options on ratios & so forth so only seem logical straightcut - semi helical & a custom mix would all be possible if asked.
Not bad kit for Aus $ price.
Any of the gearkits from PAR - Quaife - PPG can be broken if used hard & they all have been. Hard AWD launches will break anything given enough chances.
vag gen1 haldex conversion is another option if you hands on doing the development of conversion plate, mounts, flywheel, driveshafts, prop & rear diff subframe tweak. If friendly with a breaker could be cheap way of having a reasonably reliable 450-550hp transmission, plus having just a spare box would be affordable & probably achievable to convert & have a spare in under the price of 1 PAR built GTiR box if hands on & source your donor scrap carefully.

I've busted 2 quaife 3rd gears
Becker straight cut dog box
Custom built stock and dog gear set plus numerous stock boxes
Now the gtr35 box too

Think I'm just doomed with gearboxes   

Sorry for the delay. Been working like crazy.
So what constitutes a built gearbox to be safe at 400-500.
Par 1-4 and what else?

Good build, perhaps cryo treating casing.
Would be best have later model box with slightly thicker casing & larger diameter bolts on the end cover flange.
Other options are welding strengthening plates/ribs to the casing or putting a resin mould round weak point on end cover.
Going straightcut removes most risk of end case being punched out on bearing but on straightcut the shafts are being pushed apart so that stress on box could cause flex then crack.
I would still do good effort on resin moulding support around girth of end case as any help on reducing flex is a possible box & gearset saver.
Only other thing is to choose clutch wisely as using a sprung paddle with help hugely in keeping box in one piece.
Also use a strengthened clutch release arm & better quality pivot ball & then gearbox & clutch major ballache should be unlikely.

Il be doing a resin mould for my dog box soon so will do a how to with pics
We did some years back after carrying out extensive tests to casings and was going to sell them but never had the time so now il show people how to do them properly as not bothered about making money on them any more
May even put together a short vid of how to make the mould ect

If you want a bit more power why not use nitrous oxide?