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 Posted: 03-22-2015 09:40 pm
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Esprit2

 

Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 289
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Barthol wrote: I have no idea on how the bottom end is built. It was done from the Guy in Uk from who I bought the engine :-(

How do I tell whether the Crank is cross drilled and have the right bearings, is it easy to see if I open it up any way?
Can you get back to the guy you bought the engine from and ask about the crank & bearings?

Pulling the cylinder head for a rebuild has nothing to do with the bottom end. Deciding to tear it apart just to check whether the crank is cross-drilled or not is totally unrelated, and optional. A non cross-drilled crank is not a fatal flaw, and is not necessary for an engine that is street-driven in a way that doesn't result in you knowing every judge on a first name basis. True, if you plan to flog it hard as a part of your daily driving style, then a cross-drilled crank and the lower bearing shells that go with it would be better.

If the engine had individual main bearing caps, like most engines, then you could simply drop the sump, pull one cap, and take a look. However, the 9XX engines use one large Main Bearing Panel that captures all the journals at once. Removing it pretty much requires disassembling the bottom end. From what you have described, I don't think it's worth your effort. Instead, ask the guy who built it.

"IF" you do tear it down for a look, then journals 1, 2, 4 & 5 of a cross-drilled crank will have two holes in the journal, 180 degrees apart. They are the opposite ends of the one drilling that passes all the way through... you can slip a slender rod (screw driver) through it.

A non cross-drilled crank will have only one hole in those same journals. It's drilled at an angle to the rod journal.

The lower bearing shells are for a cross-drilled crank are plain. No oil hole. No oil distribution groove. That's the point of cross-drilling. It allows the surface area of the lower bearing shell to be maximized to support the piston & rod's downward thrust forces by not giving up area to oil distribution. Then the cross-drilled passage in the crank takes care of distributing the oil.

For a non cross-drilled crank, the lower bearing shells all have a central groove to distribute oil. It does it's job well, but takes away some surface area that would otherwise be available for supporting piston & rod thrust loads.

In all 907s, the upper bearing shells have both a groove and a hole.

907 with NON cross-drilled crank
Upper:
Groove/Hole - Groove/Hole - Groove/Hole - Groove/Hole - Groove/Hole
Lower:
Groove/Hole - Groove/Hole - Groove/Hole - Groove/Hole - Groove/Hole

907 with cross-drilled Crank
Upper:
Groove/Hole - Groove/Hole - Groove/Hole - Groove/Hole - Groove/Hole
Lower:
Plain ............. Plain ............. Plain ............ Plain ............ Plain

912 with NON cross-drilled crank
Upper:
Groove/Hole - Groove/Hole - Plain/ Hole - Groove/Hole - Groove/Hole
Lower:
Groove/Hole - Groove/Hole - Plain ........ - Groove/Hole - Groove/Hole

912 & 910 Turbo with cross-drilled crank
Upper:
Groove/Hole - Groove/Hole - Plain/ Hole - Groove/Hole - Groove/Hole
Lower:
Plain ............. Plain ............. Plain ............ Plain ............ Plain

Regards,
Tim Engel