IH: Engine Mechanicals - Sub-01B

Ironhead Drain Plugs

'76 and Earlier Cases

The plug on the bottom left of the photo is the primary drain plug. ¹⁾
- It is only used when draining the primary / tranny oil.
The one above it in the photo is for the transmission. ²⁾
- Since the transmission shares the same fluid with the primary, and the primary plug is lower than the tranny drain plug, there is no reason to remove this one.
The plug to the right center of the photo is the (crankcase drain plug). This is the one everyone has problems with. ³⁾
- This plug was never meant to be an actual case drain. It was put in there as a one time plug to seal off a hole used after 1972-1973 and should never be removed. ⁴⁾

Drain plugs in a '75 case ⁵⁾

Primary Drain Plug: 1/2“x20x1/2” ⁶⁾

So, if the only plug out of the three you'll ever need to remove (theoretically) is the primary drain plug, what is the purpose for the other two plugs?

Crankcase Drain Plug:
According to one theory, the hole was needed for the casting and machining process only and the plug was never meant to be removed after it was installed at the factory. The MoCo couldn't provide information on exactly how the hole was used during the manufacturing process because AMF did not keep good records and also a lot of records were lost during the transition back from AMF. ⁷⁾
Another theory is that people think the plug under the crankcase is a drain plug like you find on a car's oil pan and they remove it to drain excess oil collected in the case. ⁸⁾
- However, dry sump engines, like Harleys, drain from the oil tank, not the crank case. ⁹⁾ Draining your oil tank and refilling it there is all we need to do for changing our oil. The amount of oil sitting in the bottom of the cases is trivial, and that lower drain plug is notorious for stripping and not worth the hassle. ¹⁰⁾
- A car is a wet sump system which most people are familiar with.
- Once removed, the plug will not tighten up again because the crumbly threads have stripped out.
- Right behind the plug sits the flywheels.
- You have about 3/8“ of air space between the wheels and the end of the plug. So a tap is not going to be able to get through to establish new threads.
- The best way to fix this correctly is to dismantle the entire engine and split the cases. Then you can tap out the hole but a better fix is to weld it over.
- Most people just use JB Weld to do their fix (?) and let it go at that.

The depression in the pic below is from the boss on the plug smashing the case on over tightening.
Then, one time when the plug got removed, that piece of stressed aluminum broke free. ¹¹⁾
The same thing can happen to the threads. They can break free upon removal of the plug for the same reasoning above.


Crankcase Drain Plug on a 1967 XLCH ¹²⁾

Note that there will be no seal effect here because there is no surface for the plug boss to contact. ¹³⁾
- Oil will travel past the thread gaps and with no sealing surface, it will leak.
- That means a gasket may not effect a seal as there is not much for the gasket to squeeze against.
- In this case, some kind of pliable sealer (one that won't rip the threads out if you need to remove the plug again) smeared on the threads of the plug before installing it may work.
- If you use Teflon tape, you'll need to reduce the torque applied to the plug to keep from overloading the threads with wrench force.