Re: Oh man. HUGE mess with oil filter.
There is something else to consider here. Not sure if you guys care, but, there is a situation that can happen when the threads are machined to the wrong tolerance.
I am a precision machinist with 18 years experience. I have cut just about every thread you can imagine to the correct specs.
The most used class of thread is a class 2. Every kind of thread the average joe has ever seen or used is a class 2. The class 2 is toleranced to be a on/off type of fit. It's made to be a service-able fastener. It has the correct slop (if you will) in the pitch diameters and enough clearance in the major/minor diameters to make it come off with nominal force.
Now, Lets talk about class 3 threads. Class 3 is a interference/wrench fit thread. It will go together with a wrench and it will not back off. And believe me, heat, pressure whatever will not budge it out. This design is only used in one-time installs of critical devices. I have made components for US subs that require the class 3 fit.
There is a case, when you machine a thread out of tolerance, that the mating pieces will go on together smooth as silk. But when you try to remove the pieces, it will not budge. You can put the largest wrench you want on it. heat it, bang it, whatever. It's not coming off unless you weaken the joint with some kind of cutting. You see, the thread needs a certain amount of clearance. When the threads go together it"s moving in one direction. When you try to remove an improper fit thread, the thread mechanically locks itself against the lack of backlash. Without that bit of clearance/backlash, you basicly crush the threads into each other and the pieces will not come apart.
In this filters case. One of two problems may exist:
1. The filter"s internal thread was cut undersized (worn tap in production) and caused the mechanical lock.
2. Your engines filter mount (external thread) was machined wrong and will cause this problem every time.
If you think this is not possible because Fram and GM are so acurate.....
Think again. Because to fall out of a class 2 thread tolerance at these sizes only takes .003" to .005" (three to five thousands of an inch) and you can expect to see this phenomenom. It happens. I have seen it many times.
Johnny
