Cross drilling crankshaft on TDC

[sd_iconoclast]

I posted this a couple days ago, but I could not get on to get the answer before the last forum died.

I have a couple of '92 TDCs that I pulled out of cars in the junkyard. Both engines seemed to have spun rod bearings. I noticed that the Chevrolet power book recomends cross drilling the mains. Specifically, the book says:
"The GM Motorsports Technology Group recommends cross-drilling the center two main bearing journals on v6/60 crankshafts used in competition engines. This modification provides additional oil flow to the connecting rod bearings at high engine rpm".
I have talked to two machininsts and the both say that cross drilling the mains will weaken the crankshaft too much. Has anyone tried this? would tuftriding or nitriding help? I would appreciate any advice on the subject.

[SappySE107]

Ive read that too but I wasnt sure about the strength from doing this kind of mod. Im wondering if its necessary though. I will ask the machine shop here next week when I take my heads to be decked.

Nitriding sounds like a good idea as well but again, I have no experience with it so I don't know if its worth the effort either.

[sd_iconoclast]

I am tempted to try it for a couple reasons.
1] The chevy power book is very clear that it is talking about the stock crankshaft, and not some tougher aftermarket crankshaft.
2] Just from a general engineering standpoint, any cylinder (the main journal) that experiences twisting forces gains the overwhelming majority of its strength from the material nearest the edge of the cylinder (the bearing surface). Cross drilling would remove very little material from this area and the material on the interior of the crank contributes quite a bit less to the crank's strength.
That is my reasoning anyways. If anyone can see a flaw in it, let me know.

[SappySE107]

The DOHC crank is the same casting as the 3.1 but a different part number. I dunno if its any stronger but the features claimed in a 91 mag said polished journals and rolled fillets.

[sd_iconoclast]

The Chevrolet power book claims that all v6/60 crankshafts have rolled fillets. I will check for this next time I go to the machine shop. My machinist says he has lots of 3.1 crankshafts laing around.

OK, I just got back from the machine shop and a big talk about cross drilling. Both my machinist and a very experienced mechanic were against it as they said it would weaken the crank. They suggested using fully grooved main bearings. On the face of it, this would seem to be a solution. Why were the mains not fully grooved from the factory? Is there some benefit to having more bearing surface on the bearing half in the main cap? The mechanic of our group claims that the upper bearing half (the one in the block) is the half that takes most of the load. This does not make sense to me. It seems to me that the lower bearing half (the one in the main cap) should be the one to take most of the force on the power stroke.

I am now leaning towards buying two sets of main bearings and using only the grooved halves from each set. Anyone see any problems with that?

[614streets]

The groved bearings are a good idea versus cross drilling.

[3.4Z24]

no engine manufacturer produces an engine today with fully grooved bearings-and it's not out of expense. nascar doesn't do it. pro drag rcers don't do it. indycar doesn't do it, etc.

it was stated before that the surface of the crank pin and bearing it rides on carry the load. the resulting two holes from cross drilling do little to compromise that. using fully grooved bearings decreases the bearing surface substansially.

performance crankshaft manufacturers drill up to a one inch hole through the crank pins to lighten a crankshaft. this procedure also strengthens the crank at the pin. crossdrilling will not harm a crankshaft.

find another machinist or send your crank where they don't just turn cranks and replace bearings for stock or mild performance rebuilds.

[4spdz34]

i was under the impression that the reason one half of the bearing wasn't grooved was for oil pressure, but what the hell maybe gm teaches us wrong

[3.4Z24]

oil pressure is caused by restrictions(i.e. bearing clearances). what did gm teach you about grooved bearings and oil pressure? i'd figure more or less groove would just vary the supply of oil to the bearing and not have an affect on pressure since it still has to squeeze past the same bearing clearance restriction...

[GPXSS]

I agree with 3.4Z24, ive seen lots of performance SBC cranks where you could stick your finger through the rod throws. Anytime ive seen a crank break, it was at the mains themselves from the torque applied.. and seeings how the mains on the 981 castings are quite large (larger than a SBC, and some big blocks) I do not see anybody breaking them.

Also, the 3.1 crank that came out of my 93 GP had rolled fillets,(journals) im sure any 981 crank has them.

[bwawuz02]

in that pic of the drilled crank, which hole(s) wasn't there before?

[sd_iconoclast]

After all this discussion, I have decided to go with the cross drilled crank. It is sitting in my garage right now. If I could figure out how to post pics, I would post some pics that show what was done. If anyone wants to see pics, they are are going to have to explain to me how to post a pic.

[Bolt_Crank]

first... you need to upload it to some webspace, if you have some... if not, email 'em to drperry@cablerocket.com and I'll host 'em for awhile (I'll probably move 'em to different hosting if I need the space, lol)

and you need to type this into your post: {img]http://putyourlinkhere[/img}

change the { and } to [ and ]

[SappySE107]

Nah, just dont use the quick reply. Hit the other reply button and you will get the option to upload attachments to your post. I don't think I have an upload size limit so just keep it sane.

[Bolt_Crank]

lol... quick reply... didn't even notice the difference, lol, I should sleep, lol 6AM...

anyway.... let us know how long that crank lasts!