Why am I stripping threads and breaking screws?

[jtexas]

1979 rude 70
Lower unit drive shaft & shift shaft seals.

I've broke the threads out of 3 holes and the heads off 3 screws.

These are all 1/4-20 7/16" hex head screws.

Manual says "secure cover assembly screws to torque specified in section 2."

Torque values for 7/16" screws is 28-30 ft. lbs.

Started with a brand spanking new torque wrench. Ater the 1st 2 mishaps, took it back and exchanged it for another. Made sure I was setting it correctly. Tried it out to be sure I could detect when it broke over.

I'm taking it slow and steady but have yet to see a screw get past 20 ft lbs.

can anyone figure out what I'm doing wrong?

Another question:
My local BRP dealer says these screws are "vintage parts", but managed to scrounge one up. For the second broken one I bought stainless steel hardware from lowe's, but these are just plain inferior (break way more easily than the originals, and the thread breaks off 'em too). the lowe's screws are stamped "S30400 TRE".

Can I source this hardware elsewhere, and what would I look for?

thanks!

 

[tashasdaddy]

Re: Why am I stripping threads and breaking screws?

are they the correct hardness. are the hardware standard or rated bolts?

 

[iwombat]

Re: Why am I stripping threads and breaking screws?

30-ft-lbs is an awful lot for 1/4-20s. You sure that's not an in-lb rating?

 

[jtexas]

Re: Why am I stripping threads and breaking screws?

I don't usually bother with a torque wrench but precision seems to be a bit more critical with the drive shaft seal housing.

It's got me feeling a bit reluctant to turn a wrench on anything - I'm not too sure of much of anything anymore.
Chart on page 2-5 of the OEM shop manual:

______In. Lbs.*__Ft. Lbs.*__N-m
No. 6.......7-10.................0.8-1.2
No. 8......15-22................1.6-2.4
No. 10....25-35.......2-3.....2.8-4.0
No. 12....35-40.......3-4.....4.0-4.6
1/4".......50-80.......5-7........7-9
5/16"....120-140....10-12.....14-16
3/8"......220-240....18-20.....24-27
7/16"....340-360....28-30.....38-40
*These Torque Values apply unless otherwise specified.

tash, a couple of the OEM screws broke, but those have no markings. The Lowe's hardware just seems unsuitable for this application. I don't know from grades or hardness.....that's more or less my 2nd question - how to shop for hardware.

On the plus side, I'm becoming quite proficient at heli-coil installation.

 

[seahorse5]

Re: Why am I stripping threads and breaking screws?

I've broke the threads out of 3 holes and the heads off 3 screws.

These are all 1/4-20 7/16" hex head screws.

Manual says "secure cover assembly screws to torque specified in section 2."

Torque values for 7/16" screws is 28-30 ft. lbs.

Your mistake is thinking that a 7/16" headed screw is called a 7/16" screw.

Torque values are by thread count and diameter, not by the hex head width.

Your 7/16" headed screws have a 1/4" diameter and 20 threads per inch. In most outboard applications, the torque is only 5 to 7 ft. lbs., or 60-84 inch-pounds, NOT 28-30 ft. lbs.

 

[jtexas]

Re: Why am I stripping threads and breaking screws?

thank you!
I knew it hadda be some combination of ignorance & stupidity. When I reviewed my procedures after each incident I found some reason to think maybe I did something else wrong. I'm gonna feel kinda silly going back for one more gasket.......

guess this renders my second question moot.

 

[kenmyfam]

Re: Why am I stripping threads and breaking screws?

Your mistake is thinking that a 7/16" headed screw is called a 7/16" screw.

Torque values are by thread count and diameter, not by the hex head width.

Your 7/16" headed screws have a 1/4" diameter and 20 threads per inch. In most outboard applications, the torque is only 5 to 7 ft. lbs., or 60-84 inch-pounds, NOT 28-30 ft. lbs.

Is absolutely right. Thread diameter and pitch is what you base your tourqe settings from. Next is the grade of fastener, % of thread (tap drill size) and the receiving material (female thread)
Now you can save on helicoils !!!

 

[newtong_ware]

Re: Why am I stripping threads and breaking screws?

Most of the bolts used today have markings on the hex head. 1, 2, or more raised lines radiating out from the center of the head. These marks indicate the strength of the bolt.

Take a look at headbolts or wrist pin cap bolts and then at other more common bolts such as on a bicycle or such. You will notice some bolts or nuts will be dark, some brass colored, and others shiny. Different materials are better for some applications than others.

A decent old timer at a local hardware store can do a good job of educating you on this stuff. But if you are going to get serious about doing it yourself, it is important to know how to choose the right fastener.

Glad to know it was something simple, such as hex size vs shaft diameter, and oh, yeah, any time you are going to tork something to specs, make sure the bolts and the bolt hole are nice and clean.

 

[Lone Duck]

Re: Why am I stripping threads and breaking screws?

thank you!
I knew it hadda be some combination of ignorance & stupidity. When I reviewed my procedures after each incident I found some reason to think maybe I did something else wrong. I'm gonna feel kinda silly going back for one more gasket.......

guess this renders my second question moot.

It is also good practice to torque in 2 or three stages. I usually do it in two. Half the recommended torque at a time. And it is bad practice to click it two or three times before moving to next bolt. A slow steady pull and 1 click.