Initial_fd3s

iTrader: (2)

Hey I was woundering if you guys can help me out, I have noticed that my factory ast broke and i ordered a petit ast and it has just arrived i woundering if some one can give the steps in installing the ast and doing a coolant change aswell your help will be much appreciated.

ITR

iTrader: (1)

All the help you need can be obtained from this thread :

https://www.rx7club.com/3rd-generation-specific-1993-2002-16/big-fat-fd3s-cooling-thread-571088/

Initial_fd3s

iTrader: (2)

thanks alot that helped me tremendously. just one more question in the thread you sent me it said mazda recommends a 70% water and 30% antifreeze mix but in my Mazda long life gold antifreeze bottle it said to do a 50/50 mix with distilled water i live in canada so the temperture changes alot summer time can get really hot and winter really cold. so what mix should i do?

dgeesaman

iTrader: (7)

You will be best off with the 50/50. Definitely use distilled only.

Dave

Marty_1708

I may have overlooked it in the attached thread but where exactly does the Petitt AST mount? When I bought my FD it had the AST deleted. Now I want to put one back on but I can't seem to find where it mounts

Montego

iTrader: (1)

The stock AST mounts to the stock IC, if you have neither then you basically have to engineer a bracket (not difficult).


But in my opinion don't bother and just leave it out.

Marty_1708

Are there any benefits to just leaving it out?

Montego

iTrader: (1)

Yes. One less point of failure. I had a Pettit AST and for years I had this issue of leaking coolant every couple of months the fix at the time was to go and constantly replace my AST cap. At the time, I attributed the failure to cheap aftermarket caps but after a couple of years of dealing with that crap I realized that it really wasn't the aftermarket caps that were failing. I was the AST itself that didn't seat the cap(s) correctly so as soon as a little wear would occur, the AST would allow air to be sucked up into my coolant system and therefore cause the overflow tank to spill over. I did a little bit of research and it turned out that I wasn't an isolated incident so for a while I went back to a stock AST and the issue never occurred. Yeah go figure

But as you know a rocking a STOCK AST can have some serious repercussions and therefore I ended up deleting an AST altogether. I haven't looked back since and should have done that years ago. Seriously the only function of it is to trap air in the system. So IMO the real fix to the stock AST issue is to delete it altogether, properly burp the coolant system and never look back.

Marty_1708

Alright the owners before me just put a bolt in the lines and clamped them off. Is that sufficient enough for the delete?

Montego

iTrader: (1)

Its hard to picture it by just that description. Do you still have a line going through to the overflow tank?

Like this:

Coolant Air Separation Tank Elimination

Our coolant system is designed so that as the temperature goes up, coolant gets 'temporarily" dumped into into the overflow tank. Once the engine has cooled some it get siphoned out from the overflow and back into the engine. It's a constant process and that is why a leaky AST cap or coolant seal problem causes the coolant to overflow from the tank. Basically a leaky AST or coolant seal failure changes the pressure in the system and coolant never gets siphoned back into the engine from the overflow tank. Which in turns sets off a low coolant alarm, therefore we add coolant, then as we reach temperature only to have it be sent to the already full overflow tank (which spills over), afterwards to have it not be siphoned out by the system, which in turn sets off a low coolant alarm, therefore we add coolant, than as we reach temp.... It will make you go batty

but in any case check the link I posted as it shows the proper way to delete the AST

Marty_1708

Alright, I appreciate the help!

adam c

iTrader: (2)

The Pettit AST is a poor unit. The top is flimsy. I would send it back. The Tri Point ast is much better.

92BlackFD

iTrader: (1)

I've got the AST from Rotary Performance installed on my FD. It appears to be pretty well constructed. A buddy of mine has one on his FD too. So far neither of us have had any issues with it.

adam c

iTrader: (2)

The Rotary performance ast appears to be the Pettit ast. The problem with it is that it is made of materials that are not stout. The top that connects to the cap is made of very thin metal. Over time, it will warp. Once that happens, it wont seal. The result will be coolant overflow from the overflow tank. Many of us have experienced this problem with the Pettit ast, and didnt know it. The spout that comes off the top is also flimsy. It bends and buckles much too easily. This also leads to a bad seal.

Finally, it doesn't fit right. It doesn't sit properly in the oem mounts.

Its not a well made ast. The Tri Point ast is a much better unit, and is the same price:

AST - 93-95 RX-7 Tri-Point Engineering AST

Marty_1708

I wish I would have known this before I bought the Pettit AST.

swbrad

About 12 years and 30,000 miles ago, I replaced my stock AST with about 60,000 miles on it and put it on a shelf trusting the new aluminum AST completely. About 6 months ago I had the upper turbo coolent hose blow out on the freeway after hard acceleration from an on-ramp. I saw steam and my linear heat gage rising quickly and I coasted to the next off ramp. Had the car towed home and began the tear down. After that I had coolant leakage, low coolant, etc. The coolant pressure test didn't pick up any particular leakage. I replaced the Stant cap and that helped a bit. It was expediant to add water and soak up the over flow from the expansion tank.

6 weeks ago, I did another pressure test and found that the gasket between the block and the water pump mounting housing was leaking. Great, I thought! Something to fix. I put it on blocks until I could get to the repair. I was concerned that the overheating event on the freeway had done damage to the internal seals, and was glad to find an obvious problem. I replaced that gasket and other water hoses I could reach and put it all back together. It ran better with new plugs and the turbo piping tightened up, but still overflowed the expansion tank and didn't suck the coolant back into the engine.

The coolant system held 13 PSI overnight. I found no obvious exhaust bubbles in the coolant after a cold startup. I looked again at my reliable aluminum AST and found the upper surface pitted and distorted a little so I ground it flat again. Ran an errand and found spillage on the ground when I got back to the FD. After a little reflection, I decided that I could put pressure gages on the coolant lines around the AST and see what was really going on over time. However, common sense took me back to the shelf where my old, original equipment AST was resting. I replaced the aluminum AST with my old plastic AST and the coolant system has been operating perfectly for several days now.

I will probably buy a fresh OEM AST and cap and figure they will be good for a few years, and scrap the aluminum AST.

SA3R

Another thing that is often overlooked by a lot of people, is that on a stock plastic (crap) factory AST tank, the bottom barb has a smaller diameter orifice hole (a restrictor of sorts) in that barb.

This was engineered in at the factory by Mazda, specifically to slow the coolant down and give it time in the AST tank to purge gases and bubbles out of the coolant.

None of the aftermarket AST tanks have the smaller orifice in the bottom barb/line. Most likely a detail overlooked, or laziness on the part of the aftermarket guys.

I bought a Banzai/Tri-Point AST tank, which is the better design. The Pettit has a weak stamped steel neck which warps and does not seal. The Banzai/Tri-Point AST has a cast neck and is much better.

I then machined up the correct restrictor pill for the bottom hose of the new AST.

Dimensions are 3mm OD.
Hole size is 1.762mm ID.







This then gets pushed into the bottom AST hose, replicating the correct factory orifice size of the stock tank, and preserves the correct flow rate and gas separation of the original AST.

The other thing I would recommend is buying all the AST rubber hoses and replacing all of them when you add the new tank, along with good new hose clamps.

My car is stock twin turbos, and preserving the factory function of the stock AST while adding the reliability of the alloy Tri-Point tank was my main concern.


I am a big proponent of keeping these cars stock. But there are areas where the factory fell short trying to reduce weight or costs. The stock plastic AST tank is one of those areas. They ALL break around the middle plastic welded seam at some stage. Its a gamble that is not worth an engine rebuild. A good aftermarket alloy AST tank with a proper cast alloy neck will always be safer than the brittle plastic AST tank. The stock plastic unit is a time bomb.

Sgtblue

iTrader: (16)

From your signature it looks like you had the Pettit AST, which pretty much goes along with what adam c said about them. But thanks for posting up. And once again, he's the one to listen to.

FWIW... I'm not one to casually remove stuff like the AST that MAZDA felt necessary. But I deleted mine some years ago when upgrading the intercooler with the FC filler neck and a NPT plug. I have to say it's been pretty care-free. And it eliminates a few points of failure.

Sgtblue

iTrader: (16)

Not necessarily. This was my AST. Purchased, of all places, off Ebay from the guy who made it. Fit was perfect. One of the best I've ever seen.
Polished and machined neck:


Note the integrated restrictor:


Beautiful welds....shame you couldn't see them when installed. And beaded inlet and outlet nipples: