[ROVERNET - UK] Drain plugs & spanners

Fletcher gofanu at usachoice.net
Tue Mar 27 20:13:58 BST 2007

The plug is actually 7/16W/1/2BSF, which is in theory .007" larger than 
13/16.; Given normal tolerances, 13/16 will be a nice snug fit - very 
good on the brass plug that welds itself in! Standard 6 point spark plug 
sockets are near perfect. A typical good quality 13/16 willl be about 
.008 over nominal, and a crappy one might be .015 over. You will need to 
take a file and gently dress off any burrs from earlier gorilla-fisted 
abuse - keep all filing square and parallel! On this shallow and soft 
head, it is very wise to find the tightest fitting spanner possible, and 
then grind the end flat so that the chamfer is nearly gone; the chamfer 
can easily amount to 1/3 or more of the head depth of the plug. As a 
fine but worthwhile point:
Most good quality recent (last 25 yrs) boxes/sockets are "Flank drive" 
(Snap-on ^(TM), I think), which means that inside corners are relieved 
(no sharp internal corners) so that they don't touch the fastener. The 
"flats" are also skewed, as if it's already worn out. Some don't have 
"flats", but a rounded profile. These are far superior and more 
forgiving of corner boo-boos, and they don't cause them either.

I can send folk another short treatise on Britbolts, spanners, and how 
to easily make Brit spanners out of common ones. Email, subject: Britbolts.

DO NOT even THINK about open ends, adjustables, pipe spanners, or pliers 
of any kind - those are for beating people, and grabbing nuts of those 
who use them on beautiful and delicate parts!

Use the back corner plug, since you can tilt the car a bit to get it to 
drain completely.


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