Cracked rear iron by oil pressure sensor
 

I bought this motor about a year and a half ago for this project...pettit s5 streetported tii motor with 5k on it...for the last 2 years I have completely re-done my 91 vert..every nut and bolt top to bottom...the motor I bought compression tested with excellent numbers...I recently finished my full wire in for my elite 1500 Haltech...after trouble shooting some setrings in the computer and some trigger issues i finally got to a point ready to start it...I manually set timing today, it fired right up...I had it idling for about 60 seconds with a timing light...shut it down and noticed a good amount of oil leaking out the bottom...got the nose up in the air and fired it up one more time to try and pinpoint the source of the leak...at first I thought it was coming from the oil cooler line adapter on the rear iron, but there was actually oil above it...started the car one more time and noticed oil leaking around the npt plug in the oem oil pressure sensor hole, as well as oil leaking from where the oil pedestal attaches to the rear iron....shut it back down and pulled the npt plug and found the iron looks to be cracked on either side of the hole....after that I didn't even investigate the oil pedestal leak...

So is this a common place for a crack to be on a s5 rear iron? I would think it would take significant detonation to crack the iron, something I obviously did not achieve in my 2 minutes max of idle time....

I don't believe its common, I believe whoever put that plug in over tightened it

That's the conclusion I came to over night...the plug even with the crack was difficult to get out...it took some good torque for about 3 turns before it started to turn out easy...upon inspection noticed it's a NPT plug and it was forced in all the way....

it takes a good amount of work to crack it there, last time i saw it happen someone had managed to get an OEM oil pressure sensor to almost bottom out in that location not knowing its a tapered thread.

That' exactly what it was...a tapered thread plug bottomed out in there...like I said, it took effort just to get it out...I never payed any attention to it after I bought the motor since I wasn't going to use the factory pressure sensor anyway....looks like I'm pulling the motor...I'm going to clean it up and asses just how bad it really is..I may just patch it if it is small enough...

AFAIK ALL pipe threads, whether it be britsh, npt, or metric, are tapered threads, the taper is what seals it

Mazda RX-7's, and I think Miata's for that matter, use BSPT [ British Standard Pipe Thread ]. So if there as an American NPT pipe plug in there, that is probably what cracked it. Irons are delicate if confronted with forces they were not designed to handle. mannykiller had a similar issue with one of his front irons for his turbo oil feed.

https://www.google.com/search?q=npt+vs+bpt

Just ordered a BSPT 1/8" 28tpi plug for it...also ordered a tap....if the crack isn't any worse than it looks I'll be grooving it out 1/8" with the dremel, and doing a JB weld patch on it, cleaning out all the threads with a tap, and putting in the new plug with a bit of sealer on the threads...it will 100% hold and is way better than pulling the rear iron and replacing it...

Or, someone not only over cranked the plug in there but, cranked an NPT plug into a BSPT thread, accaserbating the force. It's easily done because they are only one thread off.