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M21 head question #728079
03/09/18 10:26 am
03/09/18 10:26 am
Joined: May 2010
Posts: 204
UK
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berniej Offline OP
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UK
I've just bought a cast iron head to replace the badly corroded ally one that came with my project. As far as I know they are interchangeable so expected them to be a similar profile.

However, there are significant differences in the combustion chamber area and I wonder if anyone could advise on if this looks like a standard iron head and what, if any, impact this might have on compression, performance etc please?

Please see images here: M21 heads

Many thanks,
Bernie


'81 T140
'62 A65
'56 SQ4
'49 M21
'62 99 Dommie
S8 project
G80 project
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Re: M21 head question [Re: berniej] #728086
03/09/18 11:26 am
03/09/18 11:26 am
Joined: Aug 2014
Posts: 1,686
OZ
Triless Online content
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Triless  Online Content
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Posts: 1,686
OZ
OK, Bernie, I'll give this a go. My experience with sidevalves has been with a '32 Matchless X and a '31 Matchless C. Now, although cast iron, the heads improved in subsequent years, with deeper finning and more efficient spark plug positioning.
Sidevalves run hot! In my opinion, the alloy head on an M21 would be an improvement in that, looking at the difference in the squish area, the alloy head would give higher compression, with enhanced cooling, so a performance ( sidevalve !? ) advantage would be procured ! By sidevalve parameters ! Also, in the pic, the alloy head still has the hole for the timing stick to be employed !

Re: M21 head question [Re: berniej] #728143
03/09/18 9:24 pm
03/09/18 9:24 pm
Joined: Oct 2007
Posts: 1,540
Auckland NZ
Ignoramus Offline
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Auckland NZ
cant say for certain but it looks like the one on the LHS has been ground out a bit with a dremel or similar ..........real hard to say of a pic


"There's the way it ought to be and there's the way it is" (Sgt Barnes)
Re: M21 head question [Re: berniej] #728146
03/09/18 9:42 pm
03/09/18 9:42 pm
Joined: Jul 2001
Posts: 2,293
Gothenburg, Sweden
Morgan aka Admin Offline

Ride safe today!
Morgan aka Admin  Offline

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Posts: 2,293
Gothenburg, Sweden
I don't see the flanges on the alloy head but I would hesitate to use the iron, It don't have a lot of compression even with the alloy. The alloy is superior for cooling.


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Re: M21 head question [Re: berniej] #728160
03/09/18 10:56 pm
03/09/18 10:56 pm
Joined: Nov 2012
Posts: 658
Isle of Wight, UK
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koan58 Offline
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Isle of Wight, UK
I'm surprised folk like WM20 haven't given an opinion, must be many of 20/21's out there?

On the matter of compression comparison, I'd suggest plugging the holes, and establishing the volumes of fluid they take. They look like "swings and roundabouts" in shape, so they may turn out to have similar volumes.

The difference seems to be primarily in squish/swirl, the alloy one looks to be more squish focussed. If it turns out to have a significantly lower volume, that would make sense with a "high performance head" for an M21.

Just thinking about alloy vs iron, I don't think the alloy will help much in terms of heat dissipation (as compared with an OHV engine) as the most vulnerable part of the sidevalve engine is the exhaust valve.
Increased compression may give a few extra "slug-power", but by increasing combustion temperature, at the expense of the exhaust valve.

I suspect that the iron will be lower compression, and will give a more pleasant and reliable ride.

The "corrosion" in the alloy head I would suggest, is damage from detonation, especially in the areas of overly hot valves.

Re: M21 head question [Re: berniej] #728183
03/10/18 3:15 am
03/10/18 3:15 am
Joined: May 2004
Posts: 3,822
Sydney Australia
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BSA_WM20 Offline
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Sydney Australia
Left head is a std M series head
Right one is either an aftermarket one or has been worked to get more combustion over the piston.
The iron head will get very very very hot


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Trevor
Re: M21 head question [Re: berniej] #728195
03/10/18 8:28 am
03/10/18 8:28 am
Joined: May 2010
Posts: 204
UK
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berniej Offline OP
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berniej  Offline OP
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Joined: May 2010
Posts: 204
UK
Many thanks for all your thoughts and replies.

I do have another bike with a '49 M21 iron headed engine in it and have experienced some of the heat it can generate.
On a classic bike rally, two-up on the hottest day of summer (well, UK summer) I had some mis-firing and found that the fuel was boiling in the float chamber - even with a tufnol spacer.

The current project is a '57 that I believe spent most of its life near the sea and everything is corroded. The fins on the original head have corroded almost completely away and although it should run cooler this one came up first so I decided to try going back to iron and see how it goes - I've heard that the sound is preferred by some?

I plugged and filled both with water and then weighed the content and each one contained 120g - so 120ml of head volume on each.

The iron one has certainly been worked and polished by a P.O. though as the hole for the timing stick is there but almost invisible on the inside of the head.

Cheers,
Bernie


'81 T140
'62 A65
'56 SQ4
'49 M21
'62 99 Dommie
S8 project
G80 project

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