Threads in thin cast iron

[Corn]

Dear All,

I just took off rear diffrential from 2016 BMW 330e for LSD diff install. While I was there I was thinking of having oil drain plug machined to the cast iron casing as it doesn't have one. However I'm having second thoughts as it seems the material thickness is around 7mm. And to machine the surface flat for gasket I assume there will be around 6mm or 1/4 inch left.. Of course there is also possibility for measuring error for the thickness but maybe not more than 1mm. So my question is, is this wall thickness enough for drain plug? Maybe going super careful with small threads like M10 and fine threads..

Thank you in advance

 

[Limy Sami]

Use a taper pipe plug say 1/8 or 1/4 (or larger if you've space) ..............BSP thread (or whatever they call it today) …..no need for a flat surface as the taper thread's the seal.

 

[Corn]

Good point. I actually have drill and tap for NPT 1/8". Then have socket head plug installed. What do you think and was this thread actually self sealing? Should I choose StSt plug or steel for the cast iron?

 

[Limy Sami]

It should self seal - if not a bit of PTFE tape or none setting pipe dope.

Personally I'd go for ordinary carbon steel - so much easier to drill out ............which of course you won't have to do if it's carbon, BOTOH - as sure as god makes little green apples you will if it's staimless

 

[morsetaper2]

If there isn't a lot of material at the loc where taper plug might be desired. Just beware that too much torque on the tapered pipe plug could split the hole. Just don't be ham-fisted w/ the threaded plug install and use teflon tape/pipe thread sealer. So you don't have to torque it hard. [ edit: Limy beat me too it...]

We just did a repair at work for that exact situation, but it was an alum housing, rather than cast iron.

 

[Corn]

That's it. Looks tiny hole, but I have time to wait for the oil to drain. Thanks everybody.

 

[Limy Sami]

That's it. Looks tiny hole, but I have time to wait for the oil to drain. Thanks everybody.
View attachment 268991

Gives you time for a cup of coffee then doesn't it

(actually - do it with the diff warm you'll be surprised how quickly a couple of litres will drain through a small hole - what weight & type of oil in it.)

 

[Corn]

Oil is 75W-90 so pretty thick. Good point to drain when warm. Should drain much faster.

Came into my mind, last night when I was on lathe trying to separate the crown wheel from the hub as it was welded instead of bolts. I don't know what they used to weld that crown wheel but it was tough as heck. I broke 3 edges of that carbide bit until I dropped the speed down to 100 rpm. Initially it was 250rpm. Diameter were 135mm so cutting speed was maybe way too high. Chip was like red glowing thread. Ok, this is bit offtopic. Sorry.

 

[Bill D]

When drain plug threads get stripped out they make rubber plugs that get jammed in the hole. There is a draw bolt down the middle to expand the rubber and seal it to the threads. very similar to the rubber test plugs for sewer pressure testing. So you do not have to even thread the hole. They work fine against a few cm of water column but not intended for real pressures
Bill D.

https://www.homedepot.com/p/Oatey-3-in-Gripper-Plastic-Mechanical-Test-Plug-33402/100168770

 

[Limy Sami]

IMO one of the most important things to check on diffs and axles are the Breathers (if fitted) blocked breathers have been the cause of many many leaking / blown oil seals.

 

[GregSY]

I use 1/8NPT all the time for diff. drains. Why? For the above reasons, it's easy to install and works well with thinner cross sections. But the other benefit is sort of the inverse of what you said....it DOES take a while to drain. It's far less mess-risky to pull a 1/8" plug and get it to drain into a pan than a 1/4" or bigger plug. Less splashing, less gushing, etc. As Limy noted, even a 1/8" plug hole will drain plenty fast.

 

[crossthread]

From your picture of the threaded hole, I would say you have plenty of meat there. Good Job.

 

[DDoug]

Braze on a thread-o-let and make it as big as you want.

 

[Hardplates]

Braze on a thread-o-let and make it as big as you want.

2" for high speed draining?

 

[Limy Sami]

2" for high speed draining?

You mean AKA - auxiliary oil supply

 

[DDoug]

2" for high speed draining?

That's the 007 feature....