Thought I'd share my frustration with other MG owners, although I doubt that
there are more than a couple to whom this will be useful information.
I spent a whole day last Saturday working on the engine rebuild of my Twincam
race engine and wound up back exactly where I started. Here's why.
The half speed shaft of the twinkie (that's the camshaft to you pushrod types,
although it doesn't obviously have any lobes in a Twincam) has it's end float
set by a flange at it's front end that runs between the block itself, and a
plate similar to the one used on the pushrod to hold the cam in the block. Now
I know everyone with a pushrod either just reuses the old plate, or at best
buys a new replacement, but just about no one actually measures what the end
float is with a dial indicator. Well in good race engine prep, you do pay
attention to all of the little details, especially as in the case of the
Twincam, this detail really matters. The distributor runs from a gear stuck
out at the front end of this shaft, rather than running off the gear in the
middle like the pushrod does. If it isn't set up right, you not only get
timing variation, you wear the gears out, and the cost of gears made from
unobtanium is pretty stiff these days.
When I measure the end float with no shims under the bolt on plate, there was
twice too much there!! After scratching my head and swearing a bit, I measured
the thickness of the gasket material for the front plate with a micrometer,
and then dug out an OEM one to compare. The OEM was about .010 thinner than
the generic replacement, and had moved the whole plate forward by that much!
The rest of my day was spent removing the front engine plate, cleaning the old
(well, new really) gasket goo off of it, making up a new gasket from a brown
paper grocery bag (they probably thought that I was demented, prowling the
aisles with a micrometer in hand measuring various bags), refitting the plate
and resetting the end float. At least the shims I happened to have worked and
I didn't have to make up more - you have to clamp shim stock between two
pieces of wood, drill the holes, and then cut the shape with scissors. (Don't
do it the other way, or you risk all of the careful cutting being wasted when
the drill picks up the thin brass and turns it into a pretzel if you haven't
clamped it properly).
So - one day down the dumper, but at least that's one step properly done. Some
time I'll tell you about setting end float on a balanced crank (on a pushrod
race engine) that I didn't want to trash just because thge thrust faces were
worn, by sticking shim stock on the backs of the thrust washers with super
glue.
Bill Spohn
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