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HondaDustR HondaDustR is offline
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Join Date: Jan 2007
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Question Need crank drilling advice

I'm soon going to have a short block to finally get to work on and I was planning on having the crank cross drilled. Of course, it is deceptively simple sounding...

Exactly how should a 944 crank be cross drilled, or perp drilled as I have seen mentioned as another option? Let me see if I can organize my thoughts well enough to follow.

1. I read in an engine blueprinting book (focuses on GM V8 builds) that they oftern cross drill the main bearings so that the oil supply to the rod journals is constant regardless of crank angle. Most main bearings have oil feed groves on only the top half of the bearing shell. The 944 had grooved mains on both halves until early 84 and then went to a standard solid lower main half. The grooved bearings cannot hold as much load, so the lowers were upgraded. The book specifies that drilling two intersecting holes at right angles will greatly help oil flow to the rod journals, since at least two oil feed holes will always be exposed to the oiling groove in the main. I assume that the V8 cranks they are dealing with have a straight through oil hole in the mains like the 944 does, but maybe they run a single hole halfway through the journal like the rod bearing holes...I don't know!

So is drilling an extra hole perpendicular to the original oil passageway through the mains on a 944 crank effective or advised? I have seen no mention of this particular cross drilling in any of the threads on the subject.

2. What I have seen in the threads (but not in the blueprinting book) is cross drilling, or perp drilling the rod journals themselves. First, what's the difference between the two techniques? The rod journals do only have one feed hole drilled to the center of the journal pin, rather than straight through like the mains. Also, it seems that the consensus is to drill only the #2 and #3 journals. Someone did a flow test on a 944 bottom end and found out that the #1 flows more than any of the others, and the #2 and 3 flow hardly any in comparison to the rest, especially in a low pressure condition. He found drilling just the 2 and 3 journals increased flow, or rather equalized the flow, but drilling all 4 did not change the original flow characteristics. Seems like 3/16" was the determined optimum drilling size.

So what's the difference between cross and perp drilling, and which way is best?

Finally, should I do the mains, the rod journals, or both? Should the mains be drilled as well as the #2 and 3 rod journals, or should just the 2,3 rod journals be drilled, and how should they be drilled?

Thanks in advance!
1987 silver 924S made it to 225k mi! Sent to the big garage in the sky

Last edited by HondaDustR; 08-31-2009 at 07:26 PM..
Old 08-31-2009, 07:23 PM
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