Well i bit the bullet and instead of trying to fix my milkshake problem with different headgaskets and sealant brands i took the engine into the shop to get it pressure tested properally.

They found a crack

The thing is that its really odd they said, they've never seen anything like this at all on engines like the A series or other motors for that matter.
The crack is waaaaay down the bore of number one cylinder and is about 15mm long hairline crack style. Just noticeable from the naked eye only when up close. Its easy to see that there has definatley been water seaping into no.1 tho, the walls of the cylinder have rusty/discoloured marks where the piston has stopped after engine turned off and the water has been sitting in the bores...

So now to fix the problem, David at the shop (great bloke who's helped me out so much) told me that they will fit a cast iron sleeve in the bore after the bore it out and leave the shoulder at the bottom for the sleeve to sit on. Supposedly it works a treat and practically gives you a new bore with more meat to bore out in the future, any one fitted sleeves to their bores?
I think i remember phunkdocta showing a photo of one a while ago...?

I thought it would be a costly exercise so i was gona give up but he quoted me about $120 - $140 for the job all up so that aint too bad!

I will get some pics tomra when its daytime!

I really have no idea how this could have happened tho, this engine has seen 5000rpm about 2 times since the twin's put and no more. Its never had a huge amount of stress put on the engine & never shown signs of overheating or symptoms like this before... Maybe it was just its time to let go

Anyway any comments at all welcome!

Cheers
Dave

Most unusual. Perhaps your block suffered from core shift when it was cast.
What oversize (If any) are you running?
I's just scrap the block and start again with a new one.
Were there any scuff marks on the piston skirt of the piston from that cylinder?

No scuff marks at all thats the weird thing...
The block has been bored 20thou over and im using slightly bigger cosworth/endurance pistons and it was working perfectly fine before i put the reground cam, twin 40's and different extractors on.
The only other thing i can maybe think of is i accidently put the A12 sump on instead of A15 (got mixed up) and ran it for about 10 minutes before i realised what i'd done, there was a slight knocking noise down below. Could this have done it ?!! surely not.....

A15's in Tassie are rare as rocking horse sh*t so i think ill take my chances with this donk! if not well its selling all Datsun parts time!

Cheers
Dave

Sleeving a cylinder bore has been an industry standard method of reclaiming a damaged cylinder for about ninety years. If it's done right, it will be as good as new, so if a replacement block can't be found for almost free, then go for it.

You probably have one of those very few blocks that had a small problem when it was cast, but all is not lost.

I hope it will be as good as new aye!
So there are some blocks that at factory when cast didnt turn out right then, how do they become deformed or how is it that these problems arise when cast?

It seems a waste of time to just throw the donk away without having a crack at fixing it as i've thrown my hard earned cash at this engine so it kind of seems fruitless to just watch it go to waste...

Cheers
Dave

the only a series ive seen with a cracked block when my friends circut a15 blew up and it was heavily worked and he drove it hard as u must have revved its tits off to crack the block on a little a series

What have you had done to this engine?

I'll just copy n paste the info from somewhere else i as i cant be feked writing it all again...

Engine:
20 thou overbored A15 (acid tubbed,honed, decked etc)
Endurance pistons 'slightly' oversize
Polished & ground crank
Fully balanced bottom end
new bearings, gaskets, seals etc
Waggot cam, grind #127D - great midrange torque and rev's to bout 8000rpm
Rollmaster adjustable cam sprocket and timing chain
Decked and slightly ported head, extra swirl job, de-dagged ports,new seats & guides
Double valve springs
lightened flywheel (was 9.8 now 5.2kg)
Twin 40mm dellortos (had them completely overhauled not 1 week ago!) on a redline manifold
Electric fuel pump feeding into some regulator i bought from the mainland - upgrading to a malpassi soon
Gemini '74 radiator - i always knew holdens were good for something!!!
Thermo fan on front of rad
Top gun leads, NGK 'v-groove' plugs
Std. points - really needs electronic ignition
GT40 perfomance coil
Datrat's Extractors
2" exhaust with sports muffler

Spent about $2500ish on the engine... But its nothing race spec. I havent revved the tits of it, the only time i can think of was 18months ago i brought it up to 7400rpm ONCE, and it liked it and didnt complain at all after that.
Apart from that, nothing at all - so most likley just a dodgy cast from factory as dodgeman said.

Cheers
Dave