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Old 05-05-2021, 12:44 AM   #1
spock345
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Default Drill and tap size for M41 OD lockout switch, M410 oil?

I need to drill out the boss on my M40's top cover so I can mount it on an OD transmission with the lockout switch. What size drill and tap am i going to need?

Also what oil should I put in an M410? SAE 30? I've also got a crap ton of Redline MTL sitting in the basement.
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Old 05-05-2021, 10:41 AM   #2
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14x2M

1/2" drill and rat tail file it out a bit to get the tap started.
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how psi stock cna support?


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Old 05-05-2021, 11:05 AM   #3
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14x2M

1/2" drill and rat tail file it out a bit to get the tap started.
So M14 with a 2MM thread pitch? Thanks. I might actually have that tap in my toolbox.
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Old 05-05-2021, 11:08 AM   #4
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Since it's pretty soft aluminum, if you're careful, you don't need to use a tap. I just went to a hardware store and bought a regular bolt with the proper thread pitch (took the switch, matched it to a nut, bought the corresponding bolt). $0.20, took it home, cut a few slots in it to clear swarf, and then carefully screwed it into the hole I'd drilled. #CHEAP

I figured I'd probably only do it once, why bother finding and buying a tap that I'd only use once?
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Old 05-05-2021, 11:09 AM   #5
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So M14 with a 2MM thread pitch? Thanks. I might actually have that tap in my toolbox.




Put the tap in a drill press to make sure it's vertical and hand feed / tap it.
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Old 05-05-2021, 11:56 AM   #6
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Originally Posted by Redwood Chair View Post
14x2M

1/2" drill and rat tail file it out a bit to get the tap started.
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Originally Posted by spock345 View Post
So M14 with a 2MM thread pitch? Thanks. I might actually have that tap in my toolbox.
It's actually M16 x 2. All of the tap charts say you should use a M14 drill but as John said the aluminum is soft so I like to use a ½" drill and let the tap remove the rest of the material for maximum thread depth. Using the drill press to start the tap straight is a good idea. Aluminum cutting oil will prevent the soft aluminum from galling.
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Old 05-05-2021, 12:11 PM   #7
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Originally Posted by hiperfauto View Post
It's actually M16 x 2. All of the tap charts say you should use a M14 drill but as John said the aluminum is soft so I like to use a ½" drill and let the tap remove the rest of the material for maximum thread depth. Using the drill press to start the tap straight is a good idea. Aluminum cutting oil will prevent the soft aluminum from galling.


Thanks it's been a while since I've done one.
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Old 05-05-2021, 12:20 PM   #8
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I happen to have the tap sitting on my desk. We got a call from another shop yesterday asking for the same info.
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Old 05-05-2021, 06:46 PM   #9
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I guess the next question is, what oil do I put in it and what should I replace while it is out of the car?

Update: I cleaned off any sawdust from the crate and popped the top off to take a look inside. Spun the input shaft and made sure everything moved as it should when going into gear. looks all intact to me, no broken or chipped teeth on the synchros. No worrying sounds from the bearings (as much as I could tell from just spinning the shaft).

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Old 05-07-2021, 11:09 AM   #10
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I stuck an unknown junkyard M41 in my PV (long time ago), and did the same thing, swapped the long shifter/short cover onto it, and eyeballed the internals while doing so, and didn't see any issues. Then found it had 99% non-functioning syncros on a couple of gears. Put up with that for a while, then pulled and took it apart. And found a couple of syncros looked fine when peered at assembled, but they were both cracked. So they'd just spread open a bit under pressure and not function properly. I was able to take the syncro rings out of my M40 to replace them.

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Old 05-07-2021, 11:53 AM   #11
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I stuck an unknown junkyard M410 in my PV (long time ago), and did the same thing, swapped the ling shifter short cover onto it, and eyeballed the internals while doing so, and didn't see any issues. Then found it had 99% non-functioning syncros on a couple of gear. Put up with that for a while, then pulled and took it apart. And found a couple of syncros looked fine when peered at assembled, but they were both cracked. So they'd just spread open a bit under pressure and not function properly. I was able to take the syncro rings out of my M40 to replace them.
I thought the synchros for the M410 and M400 didn't match the M40/41?
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Old 05-07-2021, 11:59 AM   #12
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*record scratch* Hold up, I meant M41/M40. WTF, fingers.
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Old 05-07-2021, 12:01 PM   #13
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*record scratch* Hold up, I meant M41/M40. WTF, fingers.
I am seriously wondering whether I should trade the M410 for an M41 just for the rebuild parts availability. For all my thoughts of modifying the motor I probably won't end up exceeding an M41's capacity.

This is just the first OD gearbox with a J type that landed in my lap. Well there was that other one in the east bay but that guy wanted to play the "make me want to sell it by offering an exorbitant price for an as-is gearbox" game.
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Old 05-07-2021, 12:07 PM   #14
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I'd go ahead with it since you have it. Then reevaluate *if* it has issues, which hopefully it won't. I was just saying, not all issues are very apparent even with the top off and looking carefully. I only saw the cracked syncro rings when I actually took the whole thing apart. And I wouldn't bother disassembling one unless I KNEW it had issues in the first place.
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Old 05-07-2021, 12:13 PM   #15
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I'd go ahead with it since you have it. Then reevaluate *if* it has issues, which hopefully it won't. I was just saying, not all issues are very apparent even with the top off and looking carefully. I only saw the cracked syncro rings when I actually took the whole thing apart. And I wouldn't bother disassembling one unless I KNEW it had issues in the first place.
Still kind of tempted because then I can run the stock clutch setup. I also kind of like being able to split the transmission and bell housing.

Eh, I'll pop it in and see what happens and keep an eye out for backups.

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