I have searched for this (sorry Andy if this is a bit of deja vu), but can someone please let me know exactly why it is suggested to change to the 0.6mm gasket when doing a rear head conversion on the front cylinder? Specific information is welcomed.

And Andy, I have read your theories (and others), but can I get your recommended squish (for premium pump gas - race bike use)?

I am not an expert on this but I will give it a crack.

The rear head on the MC21 has the spark plug more centrally positioned than the front head.
The front head has the spark plug screw into it at more of an angle to give clearance between the front plug and the radiator.
By putting a rear head on the front barrel, you get the advantage of a more centrally located plug,
but you have to use the "shorty" plug and cap to clear the radiator.
You also have to use a different water hose to match the different pipe connection geometry of the rear head compared to the front head.
Also, you need to change two of the head studs. This is basically all mentioned in the engine tuning section under the heading of "Supplementary modifications"

The flow of gas up into the rear cylinder is a bit of a tortured path as it has to turn through 90º after entering the crankcase in order to go up through the transfer ports.
The front cylinder is positioned pretty much in a straight line along the direction of flow through the carbs and reeds, and even between the crankshaft webs.

So possibly, it is reasonable to take from this that the front cylinder will fill better due to better flow dynamics.
Some people will say that inside the crankcase is such a turbulent environment anyway, that it has no real effect either way.
I leave the reader to draw their own conclusions here.

But lets just ASSUME for now that for whatever reason, (suggestions welcome) the front cylinder does indeed fill more efficiently.
If this is the case, with the two cylinders otherwise identical, then the compression pressures may well be slightly higher in the front cylinder.
By positioning the spark plug more centrally, then the supposedly better burn that is achieved as a result, could possibly create a situation where the front cylinder is now effectively running at a higher state of tune.
Maybe that is not the best way to describe it, but you get the idea. Maybe its all untrue? I am open to correction/ideas here that's for sure.

So just to calm things down a bit in the front cylinder, in a simple way, without getting the lathe involved, using a 0.6mm base gasket (instead of the standard 0.5mm base gasket) gives the front cylinder a slightly lower state of tune. (This hopefully is a "relative equivalent" state of tune as the rear cylinder?)
That is; 0.1mm more squish depth, a fraction more volume in the combustion chamber and hence a slightly lower compression ratio.
Note that different jetting will also help here, and I believe that if you look it up, you will find that slightly richer jetting is recommended in the front cylinder for the "rear head conversion".

So that's about the best I can do to answer the "Why 0.6mm base gasket on the front cylinder when doing the rear head conversion" question.
Hopefully someone can improve on that. And I mean factual/explanation-wise, not get-the-lathe-out-wise.

As for recommended squish,.. well that's typically a question that results in argument on this forum.
This is probably largely due to the multiple variables involved, and differing levels of knowledge/experience/personality.
I have set my MC18 SP (road use / pump gas) to 0.8mm squish with a 12.8cc combustion volume (to the the top of the spark plug hole), but am still to do a dyno chart after about 12 months!<sigh>
Others will vary, but typically 1.0mm of squish for road use (and total vague-ness on combustion volume) seems to keep most people happy.
As a comparison, stock standard squish is in the 1.2 - 1.3mm range I believe.
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[color=#808080][size=9]Yes,.. I too know how to waste Time and Money,...

Yep, stock squish tends to be around 1.2mm

R barrels, SP heads results in around 1mm and a head volume with stock head profiles that is normally considered to be pushing it's luck a bit on normal unleaded pump fuel and besides NSR's don't take well to high compression.

For info, on the NF5 if you set the head volumes to the recommended 11.5-11.7cc (which if you read the post Tuckerbag linked you can see is identical to the F3 NSR) you end up with a squish around 0.8mm.

Which agrees with conventional thinking that 0.8mm or less (if crank tolerances allow) is optimal, but this does not mean that if you have a conventional NSR setup (1.2mm squish) that you should go to R barrels, SP heads and 0.4 gaskets which would bring it down to 0.9mm as that will also remove 0.6cc from the head volume which may be (is) too much.

To test if just altering the squish makes things better or worse is a time consuming process.
Lets say you've got a motor that has a squish of 1mm and you want to test 0.8mm.
You need two more heads with 0.2mm missing from their face but with 0.4cc extra volume machined into the dome of the head itself.
Then you need to go to the dyno, run the bike, swap the heads, run the bike again.
A lot of effort for maybe no result at all.
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For what it's worth,
back in October 2011, I measured the standard squish and combustion volume of an MC18 R5k to be:
1.1mm squish
13.3cc combustion volume
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[color=#808080][size=9]Yes,.. I too know how to waste Time and Money,...

To be honest the suggestion to run the 0.6mm gasket is in some of the oldest text on the Forum, it may well just be as it's always the front that seizes if you have detonation problems and lowering the compression slightly won't hurt.
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Please do not PM me technical questions, if you can't find it on the Forum start a thread

Just to chip in, i did the mod to front head last winter using standard gasket, short plug and hrc hose. I havnt had it dyno'd and because i did it at the same time as other mods i have no idea of any gain although believe it to be marginal however reliability has been spot on with around 3k miles over the summer.

sorry to jump in here. How about mc28 conversion. Can it be used normal stock detonation and stock plugs? Front jetting increase by how many(size)?

When I raced the MC28 back in '95-'96 we went straight to the rear head on the front conversion without ever trying it with the front head. When i tried setting a 21 up with stock heads (but short barrels) we had all sorts of detonation problems on the front cylinder, totally cured by fitting a rear head.

Just look where the spark plug is on the stock front head and you'll realise how far the flame front has to travel to get to the front side of the head. It's just too far and detonation will strike before combustion is complete. The rear head still suffers, but by nowhere near as much.

Thanks for that Matt.
So basically putting the plug nearer to the centre reduces the chances of detonation, and hence, going to a thicker base gasket is actually pointless.
Sounds a lot more believable, to me, than all that speel I wrote above.
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[color=#808080][size=9]Yes,.. I too know how to waste Time and Money,...

However the HRC F3 front cylinder still uses the offset spark plug the same as the original. I've only ever used a offset head on the front of any of my bikes.
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If I have to take the carbs off once more...

Right so it would pay to use the standard base gasket (which I have so far done) and aim for a squish between say 1.1 and 1.2? Very interesting these theories...