Tapping Starter Motor Ear Holes To Eliminate Aggravating R’n’R’n The Nuts For M50 / S50 [ and other? ] Engines
In order to avoid the distasteful task of r’n’r’n the two starter bolts w/ nuts for M50 / S50 engines I am planning to tap the existing holes in the starter ‘ears’ for M12 x 1.75 bolts eliminating the issue of accessing the nuts when working with the assembly in the car.
I know this can be done in some fashion or other as several BMW specialty outfits sell starters with threaded ears, albeit not sure if they do anything special to accommodate fitment and strength issues.
The M12 bolt fits the tranny holes quite nicely, and the existing holes in the starter ears mic out at 10.6mm.
The std. hole dia. spec for tapping M12 bolts is as follows:
M12 x 1.75 - 10.2mm
M12 x 1.25 - 10.8mm
I’m not too concerned about the minimal difference between 10.2mm and 10.6mm, given that the finer 1.25 is 10.8mm, so it’s my intent to tap the existing 10.6mm for the 1.75 pitch which won’t provide the maximum rated design strength, but IMO for this application, if one doesn’t overdo the tightening torque I don’t see a problem with tapping the slightly larger hole being only 0.4mm [ 0.016" ] total, or 0.2mm [ 0.008" ] per thread, as a big deal.
Std. engagement depth for M12 x 1.75 = 0.95mm
Std. engagement depth for M12 x 1.75 = 0.68mm
Engagement depth w/ proposed set up for M12 x 1.75 = 0.75mm
That's ~80% of the std. spec. Given normal SF considerations, I jes ain't worried, leastwise for this application... but Ya'll feel free to disabuse me of my cavalier attitude re: strength, reliability concerns.
Yes, I could use the 1.25 and tap it on the slightly tight side, or go to the trouble to drill it out, but as noted I’m not really concerned preferring to use the 1.75. I do not choose to do this lightly as I have some amount of education and experience in strength of materials and structural design, not to mention considerable experience from ‘back in the day’ when we were usually ‘winging’ it with such stuff and generally found ‘slide rule’ accuracy was plenty close nuff... ';)
And yes, I am aware of the difference in 'effective total' engagement surface area differences between the various pitches, but again IMO for this and similar situations, that'd be pick'n nits...
So what say Ya’ll... about anything regarding this mod… execution, fitment, strength issues [ ears got nuff meat ], etc… any suggestions... overlooking anything that might affect any aspect of this project… especially from anyone who has actual experience either having done this or seen it in person.
TIA...
In order to avoid the distasteful task of r’n’r’n the two starter bolts w/ nuts for M50 / S50 engines I am planning to tap the existing holes in the starter ‘ears’ for M12 x 1.75 bolts eliminating the issue of accessing the nuts when working with the assembly in the car.
I know this can be done in some fashion or other as several BMW specialty outfits sell starters with threaded ears, albeit not sure if they do anything special to accommodate fitment and strength issues.
The M12 bolt fits the tranny holes quite nicely, and the existing holes in the starter ears mic out at 10.6mm.
The std. hole dia. spec for tapping M12 bolts is as follows:
M12 x 1.75 - 10.2mm
M12 x 1.25 - 10.8mm
I’m not too concerned about the minimal difference between 10.2mm and 10.6mm, given that the finer 1.25 is 10.8mm, so it’s my intent to tap the existing 10.6mm for the 1.75 pitch which won’t provide the maximum rated design strength, but IMO for this application, if one doesn’t overdo the tightening torque I don’t see a problem with tapping the slightly larger hole being only 0.4mm [ 0.016" ] total, or 0.2mm [ 0.008" ] per thread, as a big deal.
Std. engagement depth for M12 x 1.75 = 0.95mm
Std. engagement depth for M12 x 1.75 = 0.68mm
Engagement depth w/ proposed set up for M12 x 1.75 = 0.75mm
That's ~80% of the std. spec. Given normal SF considerations, I jes ain't worried, leastwise for this application... but Ya'll feel free to disabuse me of my cavalier attitude re: strength, reliability concerns.
Yes, I could use the 1.25 and tap it on the slightly tight side, or go to the trouble to drill it out, but as noted I’m not really concerned preferring to use the 1.75. I do not choose to do this lightly as I have some amount of education and experience in strength of materials and structural design, not to mention considerable experience from ‘back in the day’ when we were usually ‘winging’ it with such stuff and generally found ‘slide rule’ accuracy was plenty close nuff... ';)
And yes, I am aware of the difference in 'effective total' engagement surface area differences between the various pitches, but again IMO for this and similar situations, that'd be pick'n nits...
So what say Ya’ll... about anything regarding this mod… execution, fitment, strength issues [ ears got nuff meat ], etc… any suggestions... overlooking anything that might affect any aspect of this project… especially from anyone who has actual experience either having done this or seen it in person.
TIA...
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