750 head inlet size

Do all 750 heads except the combat have 28.5 mm ports? I have one for sale and someone asked me the port size. I've got the head securely packed and hate to unpack it. The head has a Birco casting mark but no number.

Interesting question. AFAIK, they are 30mm or 32mm (nominally). I've measured several and most are a little under 30mm or 32mm. One thing is sure - the Amal carbs were 30mm or 32mm. Not sure who made this:

marshg246 said:

Not sure who made this:

Click to expand...

It's from this document: http://atlanticgreen.com/images/cylhead.jpg
http://atlanticgreen.com/nhth.htm

Also on page 228 of Roy Bacon's Norton Twin Restoration book, and continues with:

"Carburettor adaptors used on Commando engines were:
062819 30mm from carb to port
062711 32mm from carb to port
065196 tapered from 32mm at carb to 30mm at port"

062819/*060509* should be tapered.
"CARB.INLET MANIFOLD (1 3/16-1 1/8") & STUDS (06.0509) 28/30mm"

1 1/8" = 28.575mm

* (The early 060509 manifolds/adaptors had screw-in balance pipe "Unions" 060670).
Original 062819/060509 were the multi-rib style below, not the type AN is currently selling.

Well, just to add to the confusion, this is a table I put together for my own use some years ago, with updates as I got more info. But at this point I'm not sure where some of the data came from originally, so take it for what it's worth.







Ken

May I add to the confusion? Have a 69 750 engine with just 44 stamped under steady rest and ports are 28.5 mm

When I was young and stupid, I thought bigger ports were better. The tapers on the needles are there to compensate for loss of vacuum. However the slightest increase in mixture richness can take the edge off the motor, and adversely affect throttle response. Smaller ports and slow taper needles are better. The big stuff only helps the top end. With a Commando, you need acceleration more than top end.
Rigidly mounted carbs can froth and provide rich mixture. For most riders, that does not matter.

I'm old and stupid.

I just went out and measured an early Commando head and the inlet ports are 28.5mm. Casting is sad and I can't read a single number on it. Actually new when I bought it from Raber to use while I was getting another head ported. Never really cared much for the performance with that stock head. The ported head fit my riding style. The inlet ports are 31mm on the ported head. My intake manifolds go from 35mm at the carb end tapered down to 31mm at the inlet port end. Works everywhere, although probably not quite as well as a small port head with 30mm carburetors in a parking lot.

The early 750 Commando heads are 28.5mm. These are the best heads to port because if you port upward you end up with a raised floor and better downward flow past the intake valve. The 28.5mm ports are too small for any high performance but if you port it right you get much better performance than a stock 30mm port head. Think of it as porting in the direction of a full auto port shape. See the illustration from my race manual and note that the manifold gasket is off center due to raising the port.