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MG Midget and Sprite Technical - Door striker screws

I am looking for some opinions. I am reassembling my MKII and some of the 1/4-28 flat head screws that hold the door striker to the b-post and the latch mechanism to the door are broken or missing. The normal suppliers don't seem to carry replacements.

I have found socket (allen head) flat head screws in the appropriate sizes at boltdepot.com. I can get them in a black oil finish ASTM F835 or in Stainless 18-8.

I would like to use the stainless so they don't rust but I am concerned about the strength. This is not exactly a structural application but it does take a lot of abuse. A couple of the original screws, which I assume are mild steel, are bent.

Does anyone have any better suggestion for the screws? If not what is the lesser of the two evils, rusting or bending? Do you think the stainless will hold up in this application?

Thanks,
Paul
Paul Noeth

Buy the stainless ones. I have had them on the door hinges and latches of my MGB for years. they are quite tough enough, and the socket head makes them easier to take out if you have to. Use copper grease on the threads.
Mike Howlett

Paul, you ask a very good question, because most stainless steel fasteners are weaker than carbon steel because they can not be heat treated. The safe assumption to make when looking at them is to assume "dead soft", or "grade 2" strength.

There are grades of stainless that are stronger than this, but those are typically very special and a vendor would definitely point it out if so (and are less corrosion resistant). They would not be on a hardware store shelf along with the rest of them.

The OEM fasteners for the door lock and striker were most likely grade 8, since this is a safety application (most of the fasteners on the car are grade 5).


Here is one source for them, Moss Europe:
http://www.moss-europe.co.uk/Shop/ViewProducts.aspx?PlateIndexID=1203


Since they are available new, that would be my advice, rather than the hardware store ones you were looking at.
If you never get into an accident, the strength of them won't matter. However, if you ever do have a crash and you find yourself depending on the door lock to stay latched to transfer the energy through the body and around yours, then the difference might matter a lot.


Norm

Norm Kerr

The stainless screws are attractive but they do worry me a bit, especially after reading Norm's reply.

I am not sure why I couldn't find the replacements in the Moss catalog. Both Moss Europe and Moss US have them.

Norm's comment about the strength of the fasteners in an accident is well taken. But the originals do not appear to be anything special. A couple of the screws in the door latch were obviously bent and distorted. I suppose they became loosened and were damaged by repeatedly slamming the door. I don't think a high strength screw would be so easily bent. The latch itself doesn't show any damage.

New screws are needed and since replacements are available that's the thing to do. Thank you for the input.

Paul
Paul Noeth

The stainless screws will be MORE than good enough. I defy you to break them. SS has pretty much the same UTS as mild steel depending on the grade - but it has almost double the yield strength.

I don't know what Norm is referring to with respect to heat treating. Heat treatment is normal reserved for hardening processes which actually make steels more brittle.

Stainless steel is generally "stronger" than mild steel. (that is of course using the correct usage of the word stronger)
C L Carter

The screws bend because they get loose, and then the latch bends them.
Or if really loose, the door hits them. Same screws holding the top frame, same results. Same as MGB. Use AntiSeize, get them adjusted, and tighten with a impact driver and #3 Pozi bit. Never fail or come loose again.
Worse problem is when they are loose long enough to take the threads out of the threaded plates in the body.

FRM
FR Millmore

Hi,

Thinking that heat treated steel is weaker, because it is more brittle than non-treated is a common logical faux-pas. While it is true that hardened steel does break with less elongation than non heat treated, the point at which it begins to yield is way above the tensile load where the non heat treated steel fails, so the increased brittleness is not a concern.

Here is a link to a bolt strength comparison table:

http://www.boltdepot.com/fastener-information/Materials-and-Grades/Bolt-Grade-Chart.aspx

As you can see, the Grade 2 bolt is just straight up low or medium carbon steel. The Grades 3 ~ 8 are medium carbon steel that is heat treated. Note how much higher the proof load is (the minimum point, at worst case part tolerance, considering the shape of the fastener, at which the bolt will fail).
For reference, they also show how much higher the minimum tensile strength is (the raw material properties, not considering the shape of the fastener).
The first value, proof load, is used to determine fastener torque limits when used. The second and third values, yield and tensile strength, are used by the manufacturer to control their raw material the fasteners are made of.

From the way that the yield point and the tensile strength values tend to converge, from the lowest grade fastener, up to the highest grade, CL Carter's point about brittleness can be seen, the higher grade fasteners do stretch less between yield point and ultimate tensile point. But, that "brittleness" is not a concern because note how the proof load for the next grade is always above the tensile strength (breaking point) of the lower grade.


Regarding stainless steel fasteners, here is an excerpt from a bolt supplier's website:
"It is a common misconception that stainless steel is stronger than regular steel. In fact, due to the low carbon content, stainless steel cannot be hardened. Therefore when compared with regular steel it is slightly stronger than an un-hardened (grade 2) steel fastener but significantly weaker than hardened steel fasteners."

Note that the fastener chart for 18-8 Austenitic stainless steel fasteners, it shows the raw material properties (yield strength and tensile strength), which at first seems like it might be comparable with the Grade 5 carbon steel. But look closer, there is no value given for "proof load", the key number used when designing a threaded fastener joint. Because stainless steel is more notch sensitive (threads are notches) than carbon steel, those values are lower than the raw material tensile strength would make you think. Most likely, they aren't shown on this table because they don't want them used for high strength applications.



Regarding the grade of fastener used for the door lock: most modern cars are built with seatbelt and door lock fasteners that are Grade 8 (or higher), to deal with the high, and unpredictable loading during a crash. I don't know what BMC used in our cars, but for cars built since the FMVSS seat belt and crash test laws began in '67, I have been assuming that they used Grade 8.



Norm
Norm Kerr

This thread was discussed between 19/02/2013 and 20/02/2013

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