Deck height

GaryP wrote:

Sean Trost wrote in message r.com...
Hi all !

What would be a problem if any with rebuilding a lycoming case with a
deck height out of spec.
snip

Deck height is the distance from the crankshaft centerline to the cylinder
barrel mounting surface. Machine shops that overhaul cases have a fixture
for taking this measurement. Once a case has been overhauled to the point
where it is below critical deck height several undesirable things happen.
As you mentioned, piston TDC depth in the cylinder might be so great that
it hits the head or one of the valves. Hydraulic lifter dynamic adjustment
range could be exceeded and the valve clearance not properly maintained
under all thermal conditions by the lifter. But there is also accessory
gear problems too. When the mating surfaces of a case are planed flat the
crank and cam bearing holes are made oval. These then need to be line
bored to make them perfectly round again. This effectively moves the
crank and cam centerlines outward in each case half. If there are gear
driven accessories on the back in fixed locations (e.g. mags, etc) the
gear teeth on the crank and/or cam may now be meshed to deeply with those
on the accessories. All of these issues factor in the the determination
of critical deck height and render a case beyond its usefull life.

Any reputable case overhauler like Divco or Ajax should be able to tell
you what the critical deck dimension is for your engine case.

Now if you were rebuilding a VW engine, which doesn't have an accessory
case and uses solid lifters, you would simply use cylinder barrel shims
to account for deck height. But I digress...

Gary

Concise and coherent explanation.

In fact, my only objection (at all!) is in reference to the VW engine
in the last paragraph.

There are several accessory cases available for the VW.
Most are single ignition setups, but I've seen at least one that
supported dual mags.

Thanks, Gary.
Ya done good.

Respectfully,

Richard Lamb

Richard Lamb wrote in message ...
Concise and coherent explanation.

In fact, my only objection (at all!) is in reference to the VW engine
in the last paragraph.

There are several accessory cases available for the VW.
Most are single ignition setups, but I've seen at least one that
supported dual mags.

Thanks, Gary.
Ya done good.

Respectfully,

Richard Lamb

Doh! I knew I shouldn't have digressed g.
I've never seen an accessory case for a VW beyond the Great Plains variety
which capped off the flywheel flange to accomdate a webbed ring gear for a
starter and crank driven mag. Nothing affected by deck height using this
arrangement. But if someone out there makes an accessory case for a VW
with a gear driven dual mag set up, then that is a different story and I
stand corrected.

Regards,
Gary