Will a 1959 G12 head fit on 1958 G11 engine?

Information relating to the Matchless G11 or AJS Model 30 600cc twin
Locked
Roderick
Member
Posts: 168
Joined: Mon Jan 01, 1990 12:00 am
Location: WEST YORKSHIRE UK

Will a 1959 G12 head fit on 1958 G11 engine?

Post by Roderick »

Has anyone tried fitting a 1959 type 650 cc type to a 1958 type 600 cc twin engine?

The reason I ask is that I am building up a 1958 G11 600 cc engine to install in a 1955 Spring Twin frame & have to hand both a 600 cc head & also a1959 650 cc head . The valve guides, seats, etc. on the 650 cc head are in good condition but all the kit on the 600 cc head is worn & would need replacing at considerable expense. Originality is not an issue in this rebuild; I'm just using up redundant parts that have accumulated under the bench over the last 45 years.

Comparing the two heads, both employ the same exhaust valves. The diameter of the inlet valve tulip on the 650 head is about 1/8 inch. wider than that on the 600 cc head but from looking at photos, the tops of the 'carbuncle ' pistons used on the 600 cc & 1959 650 cc heads seem the same so the wider inlet 650 cc valve may not be in danger of striking the top of the 600 cc piston. The inlet & outlet channels on the 650 cc are less obstructed by the valve guides so the 650 cc head must breathe better. Otherwise, dimensions for both heads are, as far as I can determine, the same. Therefore, the exhaust pipes & the head steady for the 600 cc should fit if the 650 cc head if it was installed.

Am I wrong or have I overlooked something?

Rod
User avatar
dave16mct
Member
Posts: 3234
Joined: Mon Jan 01, 1990 12:00 am
Location: LANCASHIRE UK

Re: Will a 1959 G12 head fit on 1958 G11 engine?

Post by dave16mct »

The 650 cc exhaust pipes are 1 5/8" diameter. 500 cc and 600 cc are 1 1/2". You could make a sleeve to fit the 600 pipes. Otherwise you'll need 650 pipes and silencers. Dave.
Roderick
Member
Posts: 168
Joined: Mon Jan 01, 1990 12:00 am
Location: WEST YORKSHIRE UK

Re: Will a 1959 G12 head fit on 1958 G11 engine?

Post by Roderick »

What I wonder is how might the 6oo cc engine actually perform with the 1959 650 cc head which takes a 1 & 1/8 inches carburettor, rather than the (1957) 600 cc head which takes a 1 inch. carburettor? Would it be over-carburretored?

An immediate practical problem is that I've discovered I would need to have a correct 1959 650 cc inlet manifold as the 600 manifold doesn't fit on the 1959 650 cc head. The 1959 manifold (larger bore size aside) looks exactly like the 600 cc manifold in that the 2 holes for the 1/4 inch. mounting studs are in a line vertically (whereas they are splayed diagonally on the 1960 on 650 cc heads) but their centres are 1/8 inch. further apart than on the 600 manifold!

Rod
Don Madden
Member
Posts: 2882
Joined: Mon Jan 01, 1990 12:00 am
Location: USA USA

Re: Will a 1959 G12 head fit on 1958 G11 engine?

Post by Don Madden »

The topic of 650cc top ends has been discussed before but I haven't searched for the links. There are three distict variadtions of the 650cc top end, 1958 only, 1959 only & 1960-up & parts should not be interchanged unless all: cylinder, pistons, heads, inlet manifold & pipes are changed as a set. Any one with a hacksaw & welding torch can fit anything to anything else but they will not work as designed. Go to Christians Archive & read my discussion titled: "72mm Pistons".

1956-58 G11: Carbuncle top pistons; 7 fins cylinders, heads have 1-1/2in inlet stud spacing & 1-1/2in exhaust ports,
1958 only G12: Pistons similar to G11 but with high pin location to accomodate longer stroke crankshaft, same cylinders, heads & inlet manifold as G11.
1959 onlly G12: New carbuncle top pistons in new 8 fin cylinders with 3in diameter top spigot; new heads with 3in recess to fit cylinders, 1-5/8in vertican inlet stud spacing to fit new manifold & 1-5/8in exhaust ports.
1960-up: New flat-top pistons, new 8 fin cylinders with 1-1/16in top spigot; new heads with matching 1-1/16in recess, angled inlet stud location & 1-5/8 exhaust ports, & different head studs; new inlet manifold to match heads.

Some of these details may seem minor but they are designed as a set & mixing can lead to poor power curve, oil leaks, blown head gaskets, etc. It is however the owners decision as what to do.

Cheers! Don the Old Curmudgeon. 8-)
User avatar
saltbox alf
Deceased
Posts: 2205
Joined: Mon Oct 03, 2011 12:25 pm
Location: Aude, France

Re: Will a 1959 G12 head fit on 1958 G11 engine?

Post by saltbox alf »

Late into early don't really go as Don says,
on the main site '60 into '59 by Chris
https://www.jampot.com/article_read.asp?id=197
Allons-y, amis de Matchless et AJS
Roderick
Member
Posts: 168
Joined: Mon Jan 01, 1990 12:00 am
Location: WEST YORKSHIRE UK

Re: Will a 1959 G12 head fit on 1958 G11 engine?

Post by Roderick »

I have carefully measured both the 600 and the 1959 650 cc heads.

They really are exactly the same in all respects (combustion chamber shape and dimensions, etc.) except that the INLET valve diameter on the 650 is 1/8 inch. larger than on the 600, its inlet port takes a 1/16 inch. larger choke carburettor and its exhaust port takes a 1/8 inch. larger diameter exhaust pipe.

Used with 600 barrels & the correct 600 “carbuncle” pistons, the only possible mechanical incompatibility of the 650 head with them that I can foresee is that its larger diameter inlet valve head might just touch the piston, which would need to be checked for.

In passing, I know that the 1960 onwards cylinder head is a different casting & a complete redesign on what went before it; higher, more fins, steeper valves, different combustion chamber, splayed studs for mounting the inlet manifold, etc. However, that's not that 650 head I'm contemplating fitting to the 600 engine.

If the capacity increase on the 1959 650 over the previous 600 twin was a mere 50 cc. & if that small increase in capacity permitted use of a 1/8 inch. larger carb & inlet valve diameter, then maybe the 600 cc engine should work as well or, perhaps, even work better with that size carb & inlet valve too?

I would really like to hear from someone who has actually done it & can report on the results they got?

And what about Sports versions of the 600 twin like the CS; what size carbs & valves were used on them?

Rod
Don Madden
Member
Posts: 2882
Joined: Mon Jan 01, 1990 12:00 am
Location: USA USA

Re: Will a 1959 G12 head fit on 1958 G11 engine?

Post by Don Madden »

Don't overlook that the inlet stud spacing for the 1959 is 1-3/4in vs 1-1/2 for G11 requiing the proper manifold & pipes are also larger. Cheers! Don. 8-)
Locked