This will not surprise anyone, but...
I noticed a coolant stain on the floor under my '93 FD. It was right under the hole in the belly pan that aligns with the OE radiator drain plug. The coolant in the reservoir tank was below the dipstick. So I stuck a Phillips screwdriver up through the pan to see if the plug was loose. I felt no resistance, so I removed the pan. The plastic plug, minus its threads, was lying on the pan, having broken off right under its head. Luckily, I had the original radiator that I had replaced in 2005 up in the rafters, so after extracting what was left of the plug up in the hole, I replaced it with the one from the old radiator.

We had just completed a ~70-mile trip the week before, and I am really lucky that it didn't totally fail until it was in the garage. So if you see a bit of coolant on the floor and/or your coolant is low check the plug.

 

Since there are still readily available full metal aftermarket options, I wouldnt see why anyone would replace with the OE when the time comes.

 

I searched online for a metal OE-style drain plug for a 3rd gen. Nothing popped up in my search that seems to be the correct piece. Do you know who currently sells them? I could use a bolt, but I'd like to have a similar head to properly retain the sealing washer.

 

My Ron Davis came with a metal plug, don't most aftermarket radiators? Earls has plugs, if we can announce the size I'm sure they're easy to source from various similar vendors. I'll try to dig it up later if no one swoops in, can't recall off top of mind.

 

Thanks, but I have the OE radiator - I guess I didn't state that clearly enough - not an aftermarket one.

 

The plug usually don't fail that way, was it over tight the first time around? Most radiator come with plastic plug, my GReddy use a metal plug but the previous Koyo use plastic plug.

If you really want to keep the OEM radiator, maybe ask Ray if Mazda carry the plug individually?

 

The plug that broke had been in there since 2007 when I replaced the OE radiator with another OE unit. And I have been the only one that has ever removed and replaced it. I thought I was being gentle enough with the Phillips screwdriver when I installed it when I changed coolant every year, but I obviously over-tightened it at some point. I have a pretty good feel for stuff like that since I've been working on my own cars for >60 years. Guess there's a first time for anything.

 

The local Mazda dealer ordered one for me and it's supposed to be in within a few days. The $12 + tax they'll charge me (no shipping cost because I'll pick it up there) is likely less than buying one from Ray and having it shipped. As I said before, I used the one from the radiator I replaced, so it wasn't an emergency.

 

If you change your coolant frequently then the plug might become brittle over time, a new one from Mazda will have you set for awhile.

 

From my engineering perspective, I don't think the plastic plug got brittle with age. What I do think is that after 15+ removals and reinstallations is that it fatigued at the stress concentration at the 1st thread of the threaded section. And as I said above, maybe marginally overtightening it didn't help.

 

Why you no upgrade radiator?

 

Because I have a real racecar (F2000) for the track, and for street use, the OE radiator works w/o issues. I had a long thread on my replacing the OE unit with a copper/brass aftermarket unit, and that replacement did not work as well as the OE unit. I also investigated getting a highly-recommended aluminum radiator, but that sounded like it would have major fitment issues with my other modifications.
Link: https://www.rx7club.com/3rd-generati...opper+radiator

 

Base model with a single oem oil cooler and oem radiator...... curious if you monitor coolant and/or oil temps? Saw on your sig the original engine....... still the case? How many miles? Quite a feat to be honest if the original engine under those circumstances. I'd be very curious to see the engine internals upon teardown.

 

Other than linearizing the stock temperature gauge, I do not monitor coolant or oil temperatures. I do pay attention to oil pressure (10W30 non-synthetic Valvoline) and it shows no sign of the oil being too hot. I only drive the FD on the street and only have ~75K on the car and engine, mostly at low load and revs except for the occasional "Italian tuneup". I expect the engine to last well past 100K under these conditions and by then I'll probably be too old to drive it.

 

Fair enough. FYI, my Ron Davis is like an exact stock replacement except metal. Rick put it for me, and it blew his mind it fit so good. He was expecting something else. Koyo fitment is a bit of a dice roll, but the fixes are known.

 

https://www.good-win-racing.com/Mazd...t/61-0537.html
I used this metal radiator drain plug on my Koyo. I suspect it might fit the stock radiator too...

 

I may order one and if it fits, great. If not, no big deal. However, the plug is $9.95, but the shipping is $17 minimum. I think I'll pass...

 

I think this is another item that is just wearing out with time more than anything else. Glad you caught it before it got bad!

Dale

 

Me too! This is not the 1st time I've caught an incipient problem (on a wide variety of stuff) just before it was ready to bite me in the butt. Guess I'm just lucky.

 

I found a few threads mentioning the same issue. It's not all that uncommon. Still can't find thread/pitch reference...yet...

Edit: Also, the problem is metal v plastic. If you move the problem down the line with a metal plug, you might just torch the plastic end tanks.

 

My thoughts also - I'd rather have to replace the plastic drain plug than the radiator. The metal plug mentioned above appears to have 2 flats on it to aid tightening/removal. Not a great idea when it's screwed into a plastic end tank.

When I go out to the garage in a few minutes, I'll mic and put a metric thread gauge on the failed one. I'll report what I find.

 

Worked on these at the dealership through the 90s. I wouldn't say this issue was common, per se (though it absolutely is with older, brittle plastic), but it wasn't rare, either. Common enough that I actually had a little plastic "bib" I would put on my drill chuck for the impending coolant shower - a slow reverse thread drill bit would usually make quick work of the issue, often without needing more than the initial bite into the plastic to spin it back out. I figure(d) it was from overtightening more than anything.

 

The thread is 10x1.25mm. The flange is 17 mm diameter.

 

I got the threaded portion out with a small flat screwdriver stuck into the hole in the middle - it was really easy. And, because the caps were still on the cooling system, almost no coolant was lost.

 

I just meant that radiators from folks like Koyo are readily available. The whole radiator, not the drain plug. Didn't mean the confusion.