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 “Building
Strength”
by
Ed Inclan
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How to prevent your 5.0L crankshaft from starving for
oil by preventing main cap walk.
When
an engine is up to temperature and spinning at 5000 RPM’s or more, the
crank and block tend to flex. This is caused because cast iron expands
with heat and moves with pressure, this can cause the main caps to walk
in the block.
The
job of the main caps is to keep the crankshaft in place and aligned.
While doing their job they get severely pounded by forces generated from
cylinder pressure. A naturally aspirated engine can produce about 1,700
lbs/psi of cylinder pressure, and if nitrous oxide is your choice,
expect about 2480 lbs/psi to be generated from a stage I system.
Taking
into consideration that the rotating assembly absorbs some of that
force, we simulated 500 lbs of pressure on a standard 302 main cap and
saw .040” deflection. This shows that given the opportunity a main cap
will deflect under pressure and could walk in the main cap saddle. If
and when this happens, vital oil pressure may be lost and metal to metal
contact will occur. Allow this to happen enough times and you will
eventually spin a bearing and the rest will be history.
The
wizards at L&R Machine found that the problem lies in the main cap
bolts. Even when using a
main girdle to strengthen the top of the main cap, the 7/16” bolt
still has .040” play at the bottom of the main cap.
This amount of slop in the main cap is what we need to get rid
of.
The
solution is to increase the size of the main cap bolt from 7/16” to ½”.
Money saving tip: (All 351W engines use a ½” main cap bolt and can be easily
used for this modification) If you are using a main cap girdle, a main
cap stud kit will be necessary.
Using
an ARP ½“stud or bolt compared to a standard stud or bolt will
increase the clamping force from roughly 12,000 psi to 19,000 psi at the
main cap. This is made possible because the standard 302 bolt gets
torqued down to 75 psi compared to 95 on a 351W and 130 psi the ARP
bolt.
This
simple modification should only be done by a qualified machine shop and
is only about $150.00. Misalignment during the modification could make your block unusable.
We
followed Bob Jones at L&R Machine during the modification process.
This modification is also available for small block
Chevy engines.
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