Pistons and Crankshaft

[Anonymous]

Hey,

In my last post I told you of my XD Project. The car is 24 years old and so i thought time for a piston and re ring change. I thought of the hyperpeutic pistons and moly ring kit. I am aware that it is a good idea to balance the crankshaft in this process, which i am going to do and add new seals and the usual, etc. However, there is much debate over lightening a crankshaft. I thought this would be a good idea to get some more acceleration and reduce the centrifugal effects causing vibration. On the falcon sixes is this a good idea and if so how much can the motor be revved safely provided the motor can breathe?

P.S. To me, as long as the flywheel is balanced and the crankshaft is also balanced then lightening it would be no problem as long as its structural integrity is not compromised.

 

[xctasy]

Hope this helps. It's a bit spooney, but its all I've garnered over the past 13 years studying these engines.

The crank should stay the same weight as it is. On a pre ED Falcon, the crank isn't fully counterweighed like the XT5 Holden and later OHC Falcon sixes are. Technically, when you take weight out of it, the torsional vibration period is lowered. On most sixes, such as Red Holdens and possibly our 250 sixes, the whole crank starts to run into a 'tunning fork rattle' at a certain engine speed. On Holdens, GM fixed this by adding either a 6 kg heavier steel crank on XU1 sixes, or a fully counterweighed XT5 cast iron crank. This pushed up the vibration period to over 7500 rpm, not the 5500 rpm that 202's ran into.

(Source, Ian Tate from a 1990 Street Machine article on Six Pot Screamers).

The trick is to do three things on any hi po six.

1. Get the best harmonic balancer(crank dampener/crank pulley) money can buy.

2. Get a eight Unbrako crank bolts (a size bigger) to secure the flywheel.

3. Knive edge any counter weights that do exist on the crank as per the standard modification David Vizard used on the old Minis. This reduces windage (oil spray separation) on crank. If you have 4 litres of oil in the sump, there could be up to 2 litres in spray at 5000 rpm, and any crank scraper, knife edgeing, and stillage basin which allows the oil to drain back will help the engine survive if you plan to lean on it.

There is a fouth thing to consider with any long stroke six:-

4. Long stroke sixes, especially the 250, had a rev range limited to 4800 rpm by Ford. The same block, short stroke 200 ran to 5500 rpm. The diffrence was oiling, windage and torsional vibration. If you want good power, the dough needing effect of the long stroke crank on oil is something to think about.

Some people have suggested using a later Pre AU crank from an OHC engine. This may be an option, but is likely to need a later ignition system and grinding to clear the camshaft. The pre BA crank had a similar oil pump arrangement, and perhaps this crank, which could spin to 5900 rpm, could be an option.

 

[Anonymous]

Cam
I polished all of the casting marks off my crank (except for a few hard to get spots) until the whole thing was smooth and had a decent shine. Took me about a full day, I then had it re-ground and balanced
Got some advise from my machinist first. I dont know how much this would effect performance at lower to mid revs (maybe none at all) but I had heard that some of the drag racers do it and I enjoyed seeing the result.