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Are the valve cover and bevel inspection cover screws M6?

I'm using a taps to clean out the threads on my cases, and I'm up to the valve covers and bevel inspection cover holes. Everything so far went as planned... I just want to do a quick sanity check with y'all to check that these are, in fact, M6 threads. I have been referring to the spare parts catalog for thread size and screw length as I don't believe much of the hardware on my 250 NC is the original. The previous owner(s) used incorrect length screws and even non-metric in a couple spots. The thing that has me confused is that most of the hardware in the spare parts catalog says something like "6MA x 40" to refer to an M6 x 40mm, however the valve cover screws say "6A x 16 UNI 2383". Is this just a M6 x 16mm? I have tried threading an M6 screw in by hand, but it doesn't really seem to want to go. Could be that the threads are just really dirty, or that the previous owner put 1/4 - 28s in there or something. Before I run an M6 tap down, I want to be sure I'm not making a big boo boo. If they are M6 then I will just run the tap down and then use Heli-coils in any of the holes that are not salvageable.

Thanks!
-Jim

Hi Jim

threads are M6 and the 16 mm lenght is correct .

Eldert

____ While the 16mm length is "correct" (as stock), it's really too short ! _ Those head-cover screws ought to be at least 18mm long.
I've posted many fine details about this concern within other past threads...
If you haven't yet read any of them, here are links to a couple related threads...
viewtopic.php?f=3&t=32&p=108&hilit=18mm#p108
viewtopic.php?f=3&t=105&p=906&hilit=18mm#p896
While the two above are not my best posts on this subject, they're all that I've found so far (for now).


Hopeful-Cheers,
-Bob

Bob's right, the standards(ASTM) are usually for structural bolted joints, and this isn't, but the rule of thumb for tapped thread engagement is, (diameter x 1.5)- my valve cover bolts were 15mm long and stuck out from the gasket 6mm, to get the required 9mm engagement would be 18mm minimum, and could be more with a steel bolt into soft cast aluminum. Everyone who fools around with cars or motorcycles should have Carroll Smith's books.
http://www.carrollsmith.com/books/index.html
Rick

I see an M6 bottoming tap in my future. Thanks all.
-j.

One more question... Anybody know what thread the front brake cable holder is? I need to do some clean out on mine...