Finally put my 1200 back together after making new engine mounts and gearbox crossmember to lower the engine and move it further back...
started it up in the shop, warmed it up, was running sweeeeeeeet, got to about 85C on the guage then pshhhhh! blows off a radiator hose!!! there is evidence of water in the oil under the oil cap.

this is my second head gasket! what's going on??? something must be wrong. maybe my guage underreads?
I haven't investigated it yet, just slammed the bonnet and pissed off home to forget about it. argh!!!

what are the symptoms of a blown head gasket?
I ask because my a15 has watery oil under the oil filler too, but not in the radiator or in the sump...

If there is any stray moisture inside the crank case, this can manifest as milky goo uner the oil filler cap, and is not nessesarily indicitive of a popped head gasket. If the crankcase is full of white milky "soup" then ther is a breach of the cooling system.
when you fired up the motor, did you idle it with the rad cap off to allow all the air pockets in the block to vent, and top up with water?
When the gasket popped last time, did you have the head skimmed? (A must with aluminiumheads) How far has the head been skimmed already? (too much compression can lead to problems)
How hot has the head been? If you get an aluminium head hot enough, it buggers the metal which means that the head bolt washers will pull iinto the head, losing all clamping force.
Or, is the head cracked? have you had it pressure tested?
Are there any bubles in the radiator? when you crank the motor does water shoot out the overflow hose?
I have enjoyed all these symptoms in days gone by.

I'll have to check again tomorrow but I'm pretty sure there's bubbles in my radiator, and it does blow water out of the overflow hose. i put this down to having a ####ty a12 radiator on an a15.
What differences in performance would i notice?

A buggered cap will leak. I was refering to full on pumps out the over flow hose when the breach cylender compresses.

I hope ur right... had me worrying for a sec.
but it also leaks out of the block in 2 seperate places...have to get them fixed

Thanks for bouncing a few ideas back, here are some answers...

A14force wrote:
when you fired up the motor, did you idle it with the rad cap off to allow all the air pockets in the block to vent, and top up with water?

Sort of.. I started it up then took the cap off to top up the water. It was full, and clean.

When the gasket popped last time, did you have the head skimmed? (A must with aluminiumheads)

Yes, the head and block were refaced, but it was not warped. Upon first inspection I didn't even notice the gasket was blown, it took an engine reconditioner to spot it. So it was pretty minor, ie no milky oil, ran on 4 cyl etc

How far has the head been skimmed already? (too much compression can lead to problems)

Pretty far, CR is just under 12:1

How hot has the head been? If you get an aluminium head hot enough, it buggers the metal which means that the head bolt washers will pull iinto the head, losing all clamping force.

Pretty hot, overheated badly once, engine half seized, but freed up when engine cooled down. I hope the head hasn't lost hardness!! It has crossed my mind but I didn't have it checked. I will this time!

Or, is the head cracked? have you had it pressure tested?

Haven't pressure tested it- but I will do that too. There were no visible cracks last time.

Are there any bubles in the radiator?

Don't know, can't tell with my ####ty radiator set up as radiator has no cap, it's in line between rad and thermostat housing. So bubbles rise and sit in top of radiator. It's a hiace van radiator and as soon as I find a better radiator it's comin out. I think it causes half my problems.

when you crank the motor does water shoot out the overflow hose?

Don't know? I'll check that out.

I have enjoyed all these symptoms in days gone by.

Have you had problems with your datsuns? Seems to me they don't normally give much trouble- except to me!
I've been plagued by overheating problems ever since I first built this car and I've had it!

I've been down this track with my a15 race motor. You will find that if you have got a comp of 12.1, you would have shaved a lot off the head/block. did you shorten your head bolts. If not you may find that the bolts will most likely be bottoming out in the block, which will not given you much torque down on the head. Have you replaced the head bolts? Over time the bolts do streach with heat e.t.c. I have replaced all my head bolts with Studs. By doing this you will insure even torque across all points of the head. The other thing to do with a comp ratio this high, is to Ring Tony Marsh in Auckland and get a Nismo head gasket. They are a lot more expensive than a standard unit, but as long as the head is torqued down correctly you will struggle to blow the gasket, unless something else is wrong.

I'm with Datoracer.
I ran a wire ringed block for years, but as soon as Tony had the nismo gaskets available he convinced me to go that way.
Worked a lot better, never blew another gasket...

aha! Thanks guys, I hadn't thought of that. We took .020 off, 10 off block and 10 off head. But who knows how much the head had already been planed. I'll pull it apart and see where I'm at. I'm still going to get this head pressure tested and change the radiator.
How much are the nismo gaskets worth?