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Old 11-30-2010, 11:54 AM
cn90 cn90 is offline
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Location: Omaha NE
Join Date: Mar 2006
Posts: 14,352
Mein Auto: 1998 528i 5-sp 140K+
Poolman idea is a VERY GOOD idea. I have done it before.

Remove the suspect bolt and match it up with an SAE bolt from a hardware store.
But the problem is the thread pitch is different for Metric vs SAE.
However, if you find a bolt with the same exact thread pitch and a "tad wider" (in terms of diameter),
it may work for you. In this case, I would torque it just a tad on the lower side, maybe a few Nm less than spec to prevent stripping.

---> The bottom line is to get a bolt just a tad "fatter". If original bolt is 8mm, then get 11/32" bolt = 8.7 mm in diameter.
Thread pitch is tricky. If the original has a thread pitch of 1.25mm (not sure but check it), then an SAE bolt with thread pitch of 20 would be fine b/c 25.4mm/20 threads = 1.27 mm, good enough for this purpose. Regardless, test this SAE bolt using an appropriate 8mmx1.25 Metric Nut.

The following is a quick guide to Metric vs SAE bolts.

Metric Bolts

Metric bolts are sized according to diameter, distance or spacing (in millimeters) between threads and length.
For example a size of 12x1.75x30 translates to a bolt that is 12mm in diameter, has a distance of 1.75 between the threads and is 30mm long.
Metric bolt sizes can also be listed as M6x1x25. This bolt is 6mm in diameter, has a distance of 1mm between threads and is 25mm in length.
A metric bolt has metric threads. Period.

Metric nuts have no identifying marks. When you purchase them try to keep them either in the original package or separated by size so you do not run into a problem later on.

Metric nut sizes are diameter x thread spacing. A 6x1x35 bolt needs a 6x1 nut.

When you see nuts listed as Ny-lok or nylon threaded it means that they have a nylon-plastic substance in the threads that keep them from loosening due to heating and cooling cycles or vibration.

SAE vs Metric Bolt Sizes

Metric bolt sizes are stated as diameter x distance between threads x length.
American or SAE bolt sizes are expressed as diameter x thread pitch x length. Thread pitch is the number of threads per inch.
An example of a SAE bolt size is 1/2x13x1. This bolt is 1/2" in diameter, has 13 threads per inch and is one inch long.
SAE bolts will not thread perfectly into a metric nut or metric threaded hole.
But there are some exceptions if you find close enough match as I mentioned.

Last edited by cn90; 11-30-2010 at 12:16 PM.
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