Friday, April 27, 2007
Drilling for Eight
Almost as soon as I posted '4-into-8' (which should immediately precede this article) I began receiving messages saying the drill-jig I've been using since the early 1970's didn't work. (!!) I even got a couple saying it didn't exist... and if it did exist, it wouldn't work. :-)
(That's it over there on the left. And just for insurance there is an uglier-but-identical home-made copy of it in my toolbox.)
What works (and exists) according to the instant experts, is a different type of drill-jig having six holes instead of eight. According to the folks who wrote, 'everyone' uses the six-hole type, pictured on the right.
As I said, I've used the 8-hole type -- successfully -- for more than 30 years and think this whole discussion is a bit lame, partly because of the assumption on which it is based but more so on the fact that, having used both types, I've found the 8-hole version to be not only more accurate, since it is indexed using four dowels, but far easier to set-up and use.
Over on the right you can see the drill-jig installed in the spigot of a cut-down flywheel.
The four new holes are drilled using a 5/16" bit, a spindle speed of about 900 rpm and a liberal dose of cutting oil. (I use SulTex.)
After drilling, the holes are sized depending on their use. If they are in the crankshaft they are reamed to 8mm; if in the flywheel they are opened up to Letter Size 'O'. The edges are carefully chamfered, the part is cleaned and you're all done.
So what's all this nonsense about the thing not working? (Or not existing :-)
As a matter of fact, it is possible to screw things up. If you put the drill-jig on the clutch-side of the flywheel, for example, the off-set hole will now be off-set in the OPPOSITE direction. Hammer as long & hard as you like, a flywheel drilled in that fashion simply won't fit.
I think that's what's behind the messages. Somebody did it wrong, blamed it on the drill-jig, and told all his buds that the thing didn't work. And if you've been on the internet for more than five minutes you're probably aware that 99% of the 'information' you'll find there is hearsay; people parroting something they've read or heard, as opposed to direct, personal experience.
When the 8-hole drill-jig is used correctly, you'll end up with a tight, precise fit. And I've got pretty good evidence it will last a long time if given a bit of care.