Christopher ,
I asked this question at rec.crafts.metalworking which can be an excellent source of
information. Following my words below I have attached the better posts. I post this to
the general Alpine list because others might find it interesting.
My opinions are :
1. The events leading up to the cam lobe destruction are sufficiently clouded as to
preclude finding THE cause of the problem. Some potential causes:
1a. Not hardening the lifter. Although most (almost all) people believe that hardened
lifters are beneficial, non hardened chilled cast iron lifters ought to be sufficient to
provide a reasonable service life. When properly prepared even non hardened lifters are
work-hardened in use.
1b. Improper break-in. Always a potential, although it sounds like you followed the
proper procedure.
1c. Lack of oil. The low oil pressure may have contributed to, or caused, a flaw
(galling) of the lifter face which lead to the failure. In any case in will be difficult
to disprove this.
1d. Bent pushrods. There are very high forces at the cam/lifter interface. The events
that lead to the pushrods being bent may have lead to a burr being raised on the lifter
face. Again impossible to know now.
Chris, I hope this is of help to you. I feel for you.
-Roger
Following are some of the posts that this group and Christopher might find interesting
and useful. One of the posts is replying to someone who suggested that spinning an
engine to prime the oil system is a bad thing and one should always prime the oil system
my some other means prior to first startup. One post that I did not included noted that
breaking in a cam/lifter is a bit of an art/science and not to be too surpised if this
process sometimes fails.
..................
Presuming that the engine is a British Leyland, Ford pushrod, etc., the lifters are in
fact
cast iron. Most people use "chilled" cast iron lifters, and the face is hardened. I've
never had them reground, but the engines I build have new parts easily available. If I
was
getting them reground I have them ground by the cam grinder.
Lifters have a slight radius ground on them. This causes the lifter to rotate as it
works.
The break-in procedure is to run the engine at 2000 - 2500 rpm for about 20 minutes
before
you let it idle. This allows the cam - which is harder than the lifter face - to wear in
with the lifter and from then on the pair are a matched set. I've often replaced lifters
on
an existing cam, but I've never used lifters with a different cam lobe than they were
originally run with.
If your initial start up bent some pushrods, then something cause the valve to stick or
bind, or hit a piston, or whatever. This will definitely cause a high shock load on the
cam
lobe, and I wouldn't be surprised if that's what caused the first problem. But wiping
off
cam lobes is probably the biggest risk of failure when starting a new engine, and many
builders have arcane rituals that they perform to exorcise the demons!
The best way to get oil pressure, as stated by someone, is to spin up the oil pump if the
engine allows that, or to externally pressurise the system if not. But I've started my
share of engines by spinning them up on the starter motor and as long as you get pressure
fairly quickly you're ok. I've certainly never damaged an engine by doing this.
Cheers, Brian
................................
For some reason I can't see the original post to this thread. However, the
theory and practice behind cam follower design and break in procedures is as
follows.
Pushrod engine type followers have a radius of about 1 metre ground on the
running face. They are also offset from the centreline of the cam lobe by
between 1 and 2 mm. The cam lobe is ground at an angle of around 3 thou per
inch to ensure initial contact towards the side of the follower. The
combination of angle, radius and offset ensures that the follower rotates to
even up wear over the entire running surface. If a follower stops rotating it
wears out in very short order.
The break in procedure is not just to "match" the follower to the cam lobe as
is commonly thought. It is also to work harden the surfaces for which cast iron
is an excellent material. The high point loadings rapidly compress the surface
skin of the lobe and follower and create a hardened skin which then resists
wear. All cam systems wear at a given rate during their service life and the
rate of wear depends on lubrication, material hardeness, regularity of surface
finish and other factors. The aim is to reach a rate of wear at which the
freshly exposed metal work hardens quick enough to resist further wear. If the
follower stops rotating or the lubrication boundary fails then wear takes place
faster than hardening takes place leading to catastrophic failure.
Modern overhead cam engines running on bucket followers use hardened steel as
the bucket material and these are ground flat not radiused although there is
still an offset to ensure rotation.
Break in procedure is to coat the running surfaces with molybdenum disulphide
lube and then run at 1500 to 2000 rpm for 15 to 20 minutes. Low speeds create
high point loadings and high speeds create rapid wear rates. Intermediate
engine speeds allow work hardening to take place without excessive point stress
on the parts.
99% of cam failures are due to inadequate break in procedures.
Dave Baker at Puma Race Engines (London - England) - specialist cylinder head
work, flow development and engine blueprinting. Web page at
http://members.aol.com/pumaracing/index.htm
.......
Checking Taylor, "The Internal Combustion Engine in Theory and Practice" --
"Flat followers acting directly on the cam surface are the simplest and
cheapest kind and have the advantage of being free to rotate in their guides.
Actually the "flat" surfaces are often slightly curved so as to allow for any
slight misalignment with the cam surface. A disadvantage of flat followers is
that pressures between cam and follower are limited to those at which the
sliding surfaces show a satisfactory freedom from wear. This consideration
places limits on spring loads lower than for roller followers. As mentioned in
Chapter 9, a good material for these parts is a special chilled cast iron.
Where cam loading is high, as in engines that run for long periods at high
piston speeds, roller followers made of surface-hardened steel and operating on
surface-hardened cams are called for. Roller followers are also advisable where
very long engine life is required, as in large marine and stationary engines."
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