I have been getting my1974 RX-4 13B engine ready to fire-up for the first time since the rebuild and have found a hole on the top, driver’s side of the rear iron (see photo). I have no idea what it is or what should go in it.

HOLE DESCRIPTION

· Vertical

· A13/64” (.40625”, 10.319mm) pin gauge will drop in the hole

· A 27/64” (.421875”, 10.716mm) pin gauge will not drop in the hole

· When the 13/64” pin gauge is inserted it drops in about 1 ¾”

· The top of the hole is threaded but only for about the first 1/4 “

· The threads look very fine but nothing I have tried seems to fit the threads

o 1/8 NPT fitting just falls right in the hole and a ¼ NPT is too big

o A Mazda water temp sensor is too small

o A M10 bolt (1.5 or 1.25) is too small while a M12 (1.25 or 1.75) is too big





I have a junk, disassembled 13B (old type but not sure of the year). It does not have the same hole but there is a boss in the same location (see photo). It just is not drilled or tapped. Looking at the boss, it is right above the coolant passageway, so my guess is the hole in by rebuilt engine goes into the coolant. My engine does have the coolant temp sensor in the driver’s side on the side of the rear iron (just like the disassembled engine).





Any idea what the hole is actually used for? Any idea on how to plug it?

Thanks for your help................

 

on the later cars that location was used for a hose barb to get water to the throttle body thermowax. i'm not aware of anything that wasn't EFI using that spot but maybe someone used it as a bleeder?

every EFI housing is like this

 

There's no RX-4 parts catalog on foxed.ca, but a plug does show up in the one for the Rotary Pick Up.
Part number is 995111200, which indicates an M12 plug. Google the number and it's still used by Mazda as a water drain plug.

 

Some one has drilled it for some reason .Just drill and use 1/4 npt tap and plug it up

 

Tap it for 1/4NPT and stick a mechanical temperature gauge bulb there.

It's what I do on all of my formerly EFI end housings. Even some 12A units... which is really nerve wracking when you are not entirely sure where to drill. But it IS curious that all of my carbureted 12A bits from before EFI have that boss right there.

 

Thanks for the info and the photo. It basically confirms that the hole just goes into the coolant so I'll just plug it.

 

Thanks................I'll probably just use a plug.

 

Will do.............Thanks

 

Great info................if the M12 plug is available I'll use that. If not I'll drill, tap and plug with 1/4 NPT................Thanks

 

I was able to get the plug (Part # 9951-11-200) and the copper gasket (Part # 9956-11-200) from my local Mazda dealer. T measured the threads on the plug and they turned out to be M12-1.50. Knowing that I was able to clean out the threads in the rear iron with a M12-1.50 tap.
Before I had the plug I was not able to accurately measure the threads in the rear iron due to its location and limited visibility. If I had known the thread size I probably would have just used a short M12-1.50 bolt with a little RTV sealant, but your help with the part number and illustration really helped.............Thanks.