Post by bra Post by ~misfit~
Mmmm shiny tractor engines!!!
Post by bra
When was the last time you looked at a pushrod?
Sonny's pushrods in
this engine are 9/16 inch in diameter.
Hollow I take it? Titanium? Anyway size isn't everything.
No, steel. The impossible chase for total rigidity in a compressed
round-section bar when valve-spring pressures reach 500lbs.
That's crazy! 500lbs valve spring pressures? That's the result of running a
pushrod engine at higher revs than the inherant design is easilly capable
Post by bra
the builders have started using solid TOOL STEEL for pushrods.
Aluminum and titanium cannot stand up to the pressures.
Again, crazy (IMO).
Post by bra
Rockers and lifters but no push rods?
The pic at the top of the first link bought back memories... I was on a
Sunday road-trip with a friend in a BMC (Austin / Morris) 1300 when things
started to go wrong in the engine. Luckily I had a few tools with me (I
always insisted on taking a small toolbag on trips longer than an hour or
two back in the pre-cell phone days) so we parked on the side of the road
and I started to investigate...
I thought about what had changed recently with the engine and knew he'd
recently had some work done by a professional <cough> shop in search of more
power - that was part of the purpose of our trip, to evaluate the modded
engine. They'd taken off the standard cylinder head and fitted it with the
head (and twin carb manifold) from a 1275 GT Mini. As well as the twin carbs
it also had higher compression, bigger valves and lighter valve gear - an
all-around better breather. They'd also fitted a re-profiled camshaft to
take advantage of
So going by that and the sound I took off the rocker cover and discovered
one of the rockers had broken off at the adjuster end, right above the valve
Here's what the original rockers looked like:
and here's the 1275 GT rockers:
As you can see the 'new' rockers were made of folded steel with threads
through the adjustment end and that's where it broke.
That seemed like an odd thing to happen and we were at least 2 hours from
home out in the sticks so I had to jury-rig it to get us back.We decided to
go home on three cylinders. I pulled the pushrod (as it was no longer held
in place by the valve spring) and, as it was an exhaust valve, also removed
the pushrod from the inlet valve of that cylinder. I'd thought about
removing the spark plug but decided to leave it in place and leave an 'air
sping' in the cylinder rather than have the thing aspirate through the
(forwards facing) spark plug hole and suck in gods know what crap.
So we headed for home at a much reduced pace but less than halfway back
another rocker broke! I pulled the rocker cover again and, knowing now that
it wasn't an isolated defective rocker got to looking carefully at the valve
gear. Crouching down and studying the geometery it became obvious that they
rockers were canted too far towards the valves. At the bottom of the stroke
with the valve fully opened they were at an almost 45 degree angle relative
to the valve! The idiot "mechanic" who'd fitted the head had used the
original push rods which were longer than the ones the 1275 GT uses due to
the thinner cylinder head. I hadn't noticed the first time I popped the top
as it was only really obvious when sitting on the ground and peering through
I ended up sitting on the side of the road pulling the rockers and springs
off the shaft and re-arraging them so that both broken rockers were over the
same cylinder and we could still run on three. I pondered my options which
weren't many with limited tools and no spares. I ended up putting the flat
washers from under the bolt heads that held the rocker gear in place between
the rocker pedestals and the cylinder head. I had to file some metal off a
couple of them to get them to fit. That obviously wasn't ideal but it lifted
the rockers a little and reduced the angle they were working on enough so
that we were able to get home on 3 cylinders without further breakage.
You can bet that I gave the mechanic an earful on the Monday!
"Humans will have advanced a long, long way when religious belief has a cozy
little classification in the DSM*."
David Melville (in r.a.s.f1)
(*Diagnostic and Statistical Manual of Mental Disorders)