In a message dated 03/08/00 9:58:03 AM Pacific Daylight Time,
matttrebelhorn@netscape.net writes:
> I'm a bit slow following up on this, but I've got some questions.
>
> First, what is "cross-drilling" the crank? How does that help? I assume
> that
> this is a "find a good race prep shop" type thing, and not an off-the-shelf
> item, yes? What are the methods and reasons for strenghtening a crank?
Well, I just did an article for an upcoming MGB mag on this, but I'll give
you the thumbnail version. Over about 6500 rpm, the stock crank will not flow
sufficient oil to some of the rods to keep the wolf at bay, so you drill
additional oil passages from the intermediate mains to the adjacent rod
journals to increase oil availability.
Strengthening? You can make sure that your grinder uses a properly radiused
stone (no sharp corners and resulting stress risers) and shot peen the
surfaces (not the journals) for the same reason. You can also nitride or
tuftride the journal surface. This doesn't really make it stronger, just
toughens the surface layer so that when you do ruin a bearing, you are less
likely to wear the crank and have to cut yet another slice off it. BTW, .020"
under is no real problem. It isn't so much a strength issue with small
undersizes, as it is a cooling question. The thicker the bearing shell, the
poorer the heat transfer. This won't bother you much on the street, but can
make a big difference on the track.
>
> So, I'd say that my motor has spent maybe 15 seconds of its life at 6500+
> RPM,
> with maybe 5 of those at over 7000. Is the crank just going to snap one of
> these days, from the repeated abuse?
I'd say not - you may have taken a few thousand miles off the life of the
bearings, but a fairly momentary blip to higher rpm shouldn't cause the crank
to crack, though it may have moved it a couple of steps closer to the day
when it will.
I doubt in fact that you actually got anywhere near 7000 rpm, as your valve
bounce rpm would be around the 6400-6500 range with uprated springs, 6200 or
so without. What did happen, was that the tach needle swept way high, but the
engine itself would likely be sitting at the valve bounce rpm, not
progressing much if any past that point.
Hope that helps.
Bill
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