Diesel Crank replacement

**smithy010** · 26-08-2006, 05:47 PM

Hi all,

I pulled the crank out of my diesel today, and realised something strange. Obviously i locked the pump and camshaft with the flywheel at top dead centre. When i pulled the sump off, i realised that 1 and 4 were at the bottom of their stroke, and 2 and 3 were at the top.... SHouldn't 1 be at TDC when the flywheel is at the tdc mark?

**smithy010** · 27-08-2006, 12:10 AM

Spot the difference between the two cranks....

That's besides the stuffed journals on main no.4 and big-end no.3

The one with red tape (bottom) is out of a 1600 petrol, and the top one is from my 1500 diesel.

The only differences i can find, is the diesel crank has unmachined counterwheights, but the petrol crank is machined all the way up the counterweights, leaving a sharper edge on the counterweights.

**smithy010** · 27-08-2006, 12:18 AM

Just a thought.....

If the pistons are differently sized from the petrol to the diesel engine, would the counterweights be of a different size?....

If the crank is balanced taking into account the piston mass, this would be true, in which case if i put the 1600 crank in the diesel, it would be unbalanced..

Anyone know more?

brackie · 27-08-2006, 06:25 AM

Originally Posted by smithy010

Just a thought.....

If the pistons are differently sized from the petrol to the diesel engine, would the counterweights be of a different size?....

If the crank is balanced taking into account the piston mass, this would be true, in which case if i put the 1600 crank in the diesel, it would be unbalanced..

Anyone know more?

Yes the crank would be balanced in accordance with the weight of the pistons and conrods (diesel and petrol use the same conrods but the pistons have flat crowns and are heavier).

No, I don't believe that the difference in mass would be significant. If you have a diesel and a petrol piston and gudgeon pin assembly out on the bench weigh them and see. Remember that it's common practice to fit flat topped pistons to petrol motors (so increasing mass) and boring out the same and fitting oversize pistons.

**smithy010** · 27-08-2006, 11:40 AM

Anyone have a diesel and petrol piston out on the bench?

I think the old crank is grindable, i've just gotta decide whether a slight imbalance will not be a problem, so it doesn't cost me the cash for a grind.

**Golf Loon** · 27-08-2006, 04:33 PM

It all looks the same smithy. I dont reckon vw would have changed anything they didnt need to. Stuff it in and see how it turns.

**smithy010** · 27-08-2006, 06:35 PM

I thought about it a little more and realised that i have two pistons on one side of the crank and two on the other side, so the crank will be totally mass balanced. There may be some slight imbalances along the shaft... (negligible, according to my advanced mechanics course last year.)

All good.

Now to get some bearings, and get this baby running!

**Edison** · 28-08-2006, 02:04 PM

so the crank will be totally mass balanced.

Not really, your thinking on a different plane, it's wobble forces would be like one end of the engine up, the other end down. The mass of the piston sortof needs it's own weight balanced as close to the piston as possible, as you transfer it along the crank you get wobbles in other planes...don't you?