Hi all,
Whilst the rivet counters would advise the cast head is 'correct' for the WM20, finding a good one is difficult. I have a 'Usable' cast head, which has only minor damage, but, is still quite rust pitted, and have wondered if there is any benefit to fitting an alloy head which can be found in good condition and finshed nicely in black to match the iron pot.
I'm asssuming the alloy head may be a little noisier (if that's possible), but is there any benefit to using one? Like, say, better heat management? Reason is, I live in Queensland Australia, and ride all year round, and we can get hot days at our historical bike Rallies, one such day was 39deg C in the morning and 42 at lunchtime whilst out riding.
The alloy head does run cooler and as you suspect transmits a little more engine noise, though not enough to worry about....
You will also need to run a 'long reach' spark plug of a different grade (Champion N5C or equivalent)...
The alloy head uses head bolts (10) that are (at least) the length of the longer ones that hold the iron head. Additionally some thick washers under the bolt heads are essential to avoid deformation of the head around the bolt holes when the bolts are tightened...Washers of approx. .080" (2mm) will do it...
Once you have the washers I would obtain some over length bolts and shorten them sufficiently so that you are using the maximum available thread in the barrel. Note that for one of the bolts the thickness of the head steady also has to be taken into account. The bolt threads are 3/8"BSF...
Use a solid copper head gasket and torque down evenly to 28/30 ft. lbs...
Check that any head you have is flat at the gasket face before use and reface it if it isn't, removing the minimum amount of material to produce a flat surface...
Also check the plug thread...I've run an alloy head on my own M20 for about the last 20 years...Ian
In regards to those head bolt washers Ian spoke about when using an alum head, drop in to your local Japanese car wrecker and pick up (literally off the ground) the head bolt washers used on alloy heads. They are very tough and wont deform. Don't pay much for them, the wrecker I collected mine from didn't charge me.
