Should i move to a duplex chain ?

[MonkeyBoy]

Hi all,

 

Getting the shopping list together for my xflow rebuild, the bit i'm really struggling with is wether or not to move to a duplex timing chain.

 

Engine is basically a std Supersprint but bored to 1720 with forged pistons, std head (though might upgrade at a later date e.g. steel posts, larger valves etc), std dizzy ignition (will move to megajolt at some point). Car is for the road with the odd track day.

 

If it was just a direct replacement then i probablly would, but as i understand you should machine the crank pulley and make a spacer for the oil thrower, or remove the thrower all together, but you still need get the crank journal machined back 6mm

 

Do i really need a duplex chain ? I'm trying to work out if it would be overkill and easiest just to stick with a single chain. I can't seem to remove the existing crank sprocket easily anyway! (use bearing puller ?)

 

Thanks for any advice

Alistair

[Roger King]

To be honest, if you're using a 234 cam and not revving over 7000rpm the single chain will normally be fine so long as you make sure you get a continuous one rather than one with a split link, and also that you use a good quality tensioner pad.

 

The material they're made from means that the standard single row sprockets are more resistant to wear than the steel duplex ones. Obviously, the duplex chain itself is stronger, but it is wise to strip and inspect every now and then to check the sprockets are OK if you have a duplex kit.

 

If you do go duplex, you need to machine the front main bearing back about .030" and to a depth of around .150". This is to stop the chain rivets fouling. You also need to put a spacer between the crank sprocket and the oil slinger for the same reason. You can easily make a spacer by removing the rubber from the inside of a 3/4" (I think - sorry it's been a long time) dowty washer and filing in a groove to accommodate the crankshaft woodruff key. The one benefit this all has is that it moves the oil slinger nearer to the front cover, so it does its job better than as standard.

 

There is another possible problem with the duplex kit. In some instances i have found that the O/D of the cam sprocket is not concentric with the centre where it mounts onto the camshaft. If you run a tensioner (it MUST be a special wide one for the duplex chain) you will find the chain goes from being properly tensioned to being as tight as a bow string and then back again when the engine turns.

 

Don't get me wrong, duplex kits can be very good, but there are a lot of pitfalls to avoid.

 

Edited by - Roger King on 20 Feb 2011 21:39:35