13 replies to this topic

[mjpuds]

ok quick queston how would i tell if my crank had been crossed drilled it is the 12g1505 model fitted to later 1275 and gt series,,with the larger end caps..it dose have holes drilled though the journals would this be the cross drilling ...also just about to finish off my 1380 rebuild..296 cam ,,gt7 head ...pushed out just over 100bhp when it had the 276 cam in so the new cam should improve this a bit.... and wondering if the main cap should have the strap fitted ,,because if i already have the larger end caps would this be as needed....

[Cooperman]

A standard crank has two holes in each journal, so it is just drilled right through once, if you see what I mean. A cross drilled crank has 4 holes, so it is drilled in the form of a cross. This is to prevent oil starvation at very high rpm's.
A centre main bearing strap is always a good idea for a high revving engine and with a 296 cam where you will be using up to 7000 rpm if you are to benefit from the aggressive cam, so cross drilling and a centre main strap would be advisable really.
Sounds like a really powerful engine.

[mjpuds]

ahhh.. and it all back together again..timed up ...torqued up all it needs is a flywheel bolt....what to do..

[MRA]

Wait a mo..... are you saying that you are using GT or 1300 rods ? on an engine that you want to rev to 8000 + rpm ??? it will last for maybe 5 minutes at those revs

Can you take a photo of the big end cap/s ??

[Cooperman]

I took 'larger end caps' to mean it has the A+ rods/caps. That doesn't mean they'll be safe for those revs without some additional work which may, of course, have been done.

Edited by Cooperman, 30 September 2011 - 06:26 PM.

[mjpuds]

sorry been away for weekend camping in new forest with fam and dub...this is the only photo i could dig up this was before i took i apart....not a great photo but as you can see only two holes in end journals..ok so now it all buttoned up i guess it would be best to open her back up and check out the crank again..

[AndyMiniMad.]

With the engine spec you have mentioned. I would seriously consider sending your crank away for some work. I think most of the engineering companys do an exchange deal with the old crank. You really want to get a cross drilled and hardened crank in that motor!! get it knifed edged while your at it. Make sure that everything else is up to spec as well...As mentioned above...rods, crank and flywheel should all be balanced as one rotating assembly.

[MRA]

Rods and pistons have to be statically balanced, as they are not rotating parts.... but reciprocal, or to put it another way if they are put on a dynamic balancer you find that they will flap around a bit

Bottom pulley, clutch built up as a complete assembly, timing sprocket, however that is such a small part as to not really cause any concern

[AndyMiniMad.]

Rods and pistons have to be statically balanced, as they are not rotating parts.... but reciprocal, or to put it another way if they are put on a dynamic balancer you find that they will flap around a bit

Bottom pulley, clutch built up as a complete assembly, timing sprocket, however that is such a small part as to not really cause any concern

Oops...Yes thats right ..My head knew what was going on, but my fingers could not keep up! I guess its an age thing...

[mjpuds]

ok opened her bac up...this is more photos..as for other work done on crank it was all balanced before i swapped the cam over..wether it has been hardened not sure

[mjpuds]

opps sorry for photo four nasty

[mjpuds]

the crank has had some work done on it as photos show been looking all over it to see if it has any signs or marking to show any other work..

[Cooperman]

As 'MRA Martin' says, the revs you will need to be using to get the performance that cam will give will be over 7300 rpm. For that you need everything balanced, the crank cross-drilled, a centre-main strap, a smaller alternator pulley. and everything else spot-on in terms of condition.
If you can't use revs as high as that you won't get the top-end performance of which the cam is capable of and designed for, but you'll lose the torque and power lower down. The 296 is a race cam for very high revs and needs a bottom end which will allow this.

[moss6273]

If I'm not mistaken, X drilling is only carried out on the big ends so you won't be able to tell if your crank has been modified this way unless you remove the big end caps!
All the other recommendations above are spot on
If anything lets go at the sort of rpm being talked about, you may be lucky and be able to reuse the front pulley and clutch (maybe drop gears) as there will be little else left which is salvageable.
An easy way to look at this is how much would it cost you to replace (to the same standard) head, block, pistons, rods, cam and gearbox. I expect you will not get much change out of £1500-2000. Against this, weigh up the cost of a fully worked - bladed, wedged, x-drilled, balanced, heat treated crankshaft. You will probably get change out of £500.00.
If you look at it this way, you can tell your partner you have just saved £1500 and have another holiday!!