27 replies to this topic

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Hi to all, 
So there is a lot of talking about the cross drilled crankshafts but not images. 
 I would like to give a go and convert my Spi crank to a Cross Drilled. 
Are there any pics that will help me or others understand, as a guide for the little details 
 

 Thanks in advance
George 

[nicklouse]

Not something easily done and not needed in most cases.

 

why do you think you need it?

[Cooperman]

You only need a cross-drilled crank for out-and-out competition use where high revs like sustained over 6500 rpm will be used. 

Otherwise it's pointless on a road car. Even my rally cars don't have cross-drilled cranks and I never have any problems.

[KTS]

In a standard mini crankshaft the oilways supplying the big end journals are drilled from the main journals.

 

at very high sustained engine speeds the centrifugal forces may cause the oil to evacuate from the big end bearings faster than it can be supplied from the main bearing oil feeds resulting in oil starvation.

 

cross-drilling involves plugging the original in-line feed drilling and drilling a new oilway at 90degrees

 

[page]

Not something easily done and not needed in most cases.

 

why do you think you need it?

 

 

You only need a cross-drilled crank for out-and-out competition use where high revs like sustained over 6500 rpm will be used. 

Otherwise it's pointless on a road car. Even my rally cars don't have cross-drilled cranks and I never have any problems.

 i' m building the engine with a BMW 16v Rs head, using odd cams. Expecting on the first stage of modifications up to 8-8500rpm (really don't know yet), then on a second stage past this point. 

[page]

In a standard mini crankshaft the oilways supplying the big end journals are drilled from the main journals.

 

at very high sustained engine speeds the centrifugal forces may cause the oil to evacuate from the big end bearings faster than it can be supplied from the main bearing oil feeds resulting in oil starvation.

 

cross-drilling involves plugging the original in-line feed drilling and drilling a new oilway at 90degrees

 

 Aha, i see the point on this, so the cross stops the oil runing fast out from the big end journals. So an non cross crankshaft can be done.
Any info of the the diameter of the cross holes? 

[KTS]

 

In a standard mini crankshaft the oilways supplying the big end journals are drilled from the main journals.

 

at very high sustained engine speeds the centrifugal forces may cause the oil to evacuate from the big end bearings faster than it can be supplied from the main bearing oil feeds resulting in oil starvation.

 

cross-drilling involves plugging the original in-line feed drilling and drilling a new oilway at 90degrees

 Aha, i see the point on this, so the cross stops the oil runing fast out from the big end journals. So an non cross crankshaft can be done.

Any info of the the diameter of the cross holes? 

 

 

no idea i'm afraid, but if you're looking to build something that'll be seeing 8500rpm and beyond you may want to consider using something other than a stock crank

[page]

 

 

In a standard mini crankshaft the oilways supplying the big end journals are drilled from the main journals.

 

at very high sustained engine speeds the centrifugal forces may cause the oil to evacuate from the big end bearings faster than it can be supplied from the main bearing oil feeds resulting in oil starvation.

 

cross-drilling involves plugging the original in-line feed drilling and drilling a new oilway at 90degrees

 Aha, i see the point on this, so the cross stops the oil runing fast out from the big end journals. So an non cross crankshaft can be done.

Any info of the the diameter of the cross holes? 

 

 

no idea i'm afraid, but if you're looking to build something that'll be seeing 8500rpm and beyond you may want to consider using something other than a stock crank

 

  It is a money limit for now on an experiment. There are a lot of money spend on other aerias too (gearbox, engine block, etc). For sure on the second stage the crnk will be replaced. 
Thanks for the info tho. 

I really appreciate if anyone knows the diameter of holes on the big end on crossed crank. 

[imack]

Think they're the same diameter as the standard ones on my old modified MED crank.

[Spider]

Careful what you wish for here !

It's done for very high reving competition engines where longevity of the crank beyond a racing seasons isn't considered. I've seen plenty of them crack  & break right through the cross drilling. The crossing drilling is supposed to slow the oil flow through the crank so the Mains are not staved of oil.

Also, the oil film between the crank and the bearings rely on an oil wedge being formed for lubrication - it has next to nothing to do with oil pressure. With most cross drilled cranks at lower revs, this oil wedge doesn't form very well, resulting in short life for the big end bearings.

If it's a road car - don't.

[DeadSquare]

This is a very good example of "If it ain't broke, don't fix it".

 

Unless the engine is for Motor Racing, (and I'd add, on circuits with long straights), it is not going to be revving at 8+k for more than a few seconds through the gears.

 

It is more important to have plenty of oil passing over the bearings, removing heat generated by compression.

[page]

It isn't a roadgoing car. Hill climb and track use. 
Plenty of bearing clerance on both Main (4bolt main cap) and Big end using Acl black bearings and all off shorts of tricks plus a 10-60 oil will bw used. Although i'm not sure still if i m using the right pump tho. Purchased the turbo one from Med, in the case of a chance of having a bit more capacity moving (did i do right?).
 Head is twice the size of an A series is that it worries me (and chossed the dry deck solution). 
Will been using close ratio gear set, 105mph@8000 as calculated. 
 

Aand you know it's first time i'm doing this so i have some second maybe third thoughts if im doing it right. im a human after all so i share my worries such as if there is a need of Cross Drilled crank. But i see yr point here.. it is tricky to decide.  

[page]

i'll go with non cross drilled and see the results from there

[DeadSquare]

i'll go with non cross drilled and see the results from there

An awful lot of the fun of motor-sport, is just taking part.  Enjoy your BMW 16V for a couple of years, and if something goes wrong, or better turns up, you have something to move on to.

 

I was never envious of pot-hunters in their Ferrari, who led the race all the way and collected a useless trophy.  Better to have bought solid silver quart tankards and quaffed bubbly on the terrace with friends, watching the sun set.

[Ethel]

The turbo pump is a good choice. Whichever you have it can only push the pressure relief valve open. It'll be easier to drill a crank if needed rather than undrill one that isn't.