Hints and Tips - Alloy Heads and Magneto Plug Gaps

From: Alan Fairless
Sent: 12 June 2000
To: british-cars-pre-war
Subject: A7 Aluminium Cylinder Heads

Does anyone know how I can get in touch with Alan Raeburn, who used to sell Aluminium Cylinder heads for Austin Sevens ( Ulster replicas) , and does he still do them

Thanks

Alan


From: Andy
Sent: 12 June 2000
To: Alan Fairless; british-cars-pre-war
Subject: re A7 Aluminium Cylinder Heads

Alan, Alan Raeburn's details are on my parts page.
--
Andy

Andy's Austin Seven Page
http://WWW.GEOCITIES.COM/MotorCity/4752/parts.html


From: Barry Lovelock
Sent: 12 June 2000
To: Alan Fairless
CC: british-cars-pre-war
Subject: Re: A7 Aluminium Cylinder Heads

Alan,

I just posted a reply on Bruce's site:-

Aluminium, Cast Iron and Bronze (unblown) Ulster pattern heads are availble from Alan Raeburn, at approx 155, 185 and 203 each respectively. They are excellent quality, VSCC accepted and use, I believe, 18mm plugs. Whether they are better in terms of performance is difficult to answer. Alan can be contacted on 01342 850422.

Barry Lovelock.


From: Graham Orme-Bannister
Sent: 22 June 2000
To: Barry Lovelock
CC: British-Cars-Pre-war
Subject: ALUMINIUM CYLINDER THREADS

Picking up a new thread from the copper head gasket debate from Barry Lovelock's note, the whole point about going to an aluminium cylinder head is to be able to run a higher compression ratio. It is an extension of the cast iron to aluminium piston story which I have been researching.

The perceived wisdom is that the main benefit of aluminium pistons was lower weight leading to lower inertia forces leading to higher engine speeds. While this is a benefit it is not the main benefit; the main benefit is much higher thermal conductivity leading to lower piston crown temperatures making it possible to raise the compression ratio. The aluminium cylinder head is the next stage in the same argument.

The down side is that much more heat is being dumped by the engine leading to bigger water pumps, cooling fans and radiators. Higher specific power outputs are achieved at the expense of lower overall thermal efficiency. Going back to the beginning there is no point in fitting an aluminium cylinder head unless the compression ratio is raised to take advantage of reduced combustion chamber temperatures.

Graham Orme-Bannister
Alresford, Hampshire, UK


From: Barry Lovelock
Sent: 22 June 2000
To: Graham Orme-Bannister
CC: British-Cars-Pre-war
Subject: Re: ALUMINIUM CYLINDER THREADS

Yes, I agree! The point being made was that the 'blown "works" engines actually suffer from a 15% power reduction with identically machined aluminium versus cast iron and possibly, bronze too. Whilst the higher compression ratio idea is ok for other engine designs, as it is fairly commonly known, the limits for side-valve Austin Seven engines are dictated by the inherent combustion chamber shape and plug placement; in other words, increasing the compression ratio will have a negative effect above a certain critical amount,

Barry.


From: Adrian Roger Twelvetrees
Sent: 22 June 2000
To: Graham Orme-Bannister; Barry Lovelock
CC: British-Cars-Pre-war
Subject: Re: ALUMINIUM CYLINDER THREADS

Graham,

> The perceived wisdom is that the main benefit of aluminium pistons was lower
> weight leading to lower inertia forces leading to higher engine speeds. While
> this is a benefit it is not the main benefit; the main benefit is much higher
> thermal conductivity leading to lower piston crown temperatures making it
> possible to raise the compression ratio. The aluminium cylinder head is the
> next stage in the same argument.
>

That is fascinating. Is that why cast iron pistons in alloy cylinders, (as in the original Buick/Rover V8) were never successful?

Roger


From: Alan Fairless
Sent: 23 June 2000
To: british-cars-pre-war
Subject: Re: ALUMINIUM CYLINDER THREADS

All this seems to be getting rather complex, and is one of the reasons us car designers get paid so little for doing so much!

Just to throw more fat on the fire, aluminium's higher thermal conductivity gives the possibility to avoid local hot spots in the combustion chamber walls these cause pre- ignition and can adversely affect the flame travel. In fact, you can have more than one flame inside the cylinder which is not good for efficiency. Whether this is of any benefit at all in an Austin 7 sidevalve is debateable. If you consider Barry's point about the power loss then the only reason for using alloy heads on A7s appears to be the ability to cast decent combustion chamber shapes cheaply.

In fact , in the 1930's there was a process called copperising, which deposited a thin layer of copper on the suface of the combustion chamber. This gave the conductivity to avoid the local hot spots, but did not radically affect heat transfer into the cooling system.

Alan


From: Graham Orme-Bannister
Sent: 23 June 2000
To: roger twelvetrees
CC: British-Cars-Pre-War
Subject: Re: ALUMINIUM CYLINDER HEADS

Roger,

I am not familiar with the Buick/Rover V8 in detail, I cant afford a Range Rover, and I was not aware it originally had an unsleeved aluminium block and cast iron pistons. The starting point for this I guess is that you could not run aluminium pistons in an aluminium bore - bad bearing combination.

Some years ago industry forecasts were very strong on the all aluminium engine with no cylinder liners. The aluminium for the block needs a very high silicon content, something around 12% as I recall, to give the necessary wear resistance. The buzz word was "hyper-eutectic silicon alloys". The pistons were usually aluminium but with a surface coating, such as hard anodising. Lightweight cast iron pistons might be OK in a softly tuned engine; maybe that is where the Rover V8 went wrong because people started to tune it to degrees that I am sure Buick never intended. What cylinder/piston metallurgy does the current Rover V8 engine have ?

Back to matters pre-war. The magneto on my Riley packed up recently after only a few hundred miles, but quite a long elapsed time, since the engine rebuild. Fortunately I had a spare and while I was at it I changed the plugs and HT leads. When I took the old plugs out the gaps were some 30 thou., I set the new plugs at the recommended 18 thou. and the previously slow cold starting miraculously became instantaneous. Now to the point; reading some recently aquired copies of early Riley Register Bulletins I came across a reprint of a 1930's Riley Technical Bulletin to the effect that a common cause of magneto failure was excessive plug gaps putting too high a resistance load on the magneto windings leading to overheating and failure. It fits my situation exactly but I had never heard this before. Do the electrical experts in our number, of which I am not one, accept this thesis.

Graham Orme-Bannister
Alresford, Hampshire, UK


From: Adrian Roger Twelvetrees
Sent: 23 June 2000
To: Graham Orme-Bannister
CC: British-Cars-Pre-War
Subject: Re: ALUMINIUM CYLINDER HEADS

Graham,

The original target for the Buick engine was supposedly an outboard motor. It failed at that and was then picked up for a US compact car, with the design converted to conventional liners. I think that the compact was a failure and Charles Spencer King was able to licence that design for Rover because there were no other outlets for it.

On the topic of magneto failure I am extremely sceptical about a largish plug gap damaging the insulation. There should be a safety spark gap somewhere on the high voltage side, usually under the rotor arm, which sparks before the voltage rises to excessive levels. The energy dissipated in the magneto might be expected to be less if the plug gap is larger, since the current will flow for a shorter time. In any case, the energy involved must be tiny compared to the conducted heat from the engine. Older magnetos fail from ageing of the enamel on the wire, or breakdown of the insulating tape between the layers of wire. This is a well known "life" effect of older insulating materials, which can be delayed by keeping the winding cool and perfectly dry. There should be no such failure mode in modern materials, and the problem is almost certain to be a breakdown of the capacitor. So called infant mortality is a problem with electronics. This is overcome on high quality electronic equipment by "stress testing" at high temperature to weed out incipient failures before shipping to the customer. If the winding has broken down, then the reason is likely to be a quality failure in the layering and interleaving of the high voltage winding. Independent ignition supplies will supply you with a capacitor by mail order for a few quid if you feel confident enough to fit it yourself.

Best wishes,

Roger


From: Peter Jacobs
Sent: 23 June 2000
To: Graham Orme-Bannister
CC: British-cars-pre-war
Subject: Re: ALUMINIUM CYLINDER HEADS

Graham,

Regarding your magneto, if the plug gap is too wide then the voltage generated in the magneto has to be greater to the make the spark jump the gap. The danger is that it may find an easier path by jumping between layers of the fine wire windings in the magneto's coil. Once this has happened the insulation may be broken so that it always chooses this path even when the plug gaps have been narrowed. This means a rewind is necessary. The coil's insulation can be degraded by prolonged exposure to damp. The best place to keep your mag's when the car is off the road is in the airing cupboard!

It is usual for magnetos to contain a "safety spark gap". This is designed to permit the spark to jump to earth in circumstances such as a plug lead becoming disconnected, and is set so that the magneto's coil is not overstressed. Unfortunately, these safety gaps are not universally understood, and people widen them in conjunction with wider plug gaps to get a bigger spark at the plug. It may work for a while, but in due course the coil insulation will give way.

Therefore, always maintain plug gaps at no more than 18 thou when a magneto is in use, and regularly check that there is minimal electrical resistance between the each plug terminal and its segment in the distributor cap. Do not use anything but copper cored HT leads, and avoid plug caps which incorporate interference suppressors.

Hope this helps.
Peter


From: Adrian Roger Twelvetrees
Sent: 24 June 2000
To: Richard Weid
CC: Graham Orme-Bannister; British-Cars-Pre-War
Subject: Re: ALUMINIUM CYLINDER HEADS

Graham,

Gee, what an amazing resource the lists is!!!

Roger


From: David Whittle
Sent: 26 June 2000
To: British-Cars-Pre-War
Subject: Re: ALUMINIUM Blocks

They should surely have hard chrome plated the inside of the bores and used ally pistons. NSU mopeds were done this way in the 50s with great success. Cast iron pistons sound archaic by the time this engine came along?

Regards
David Whittle


Home | Hints & Tips | Suppliers | Engineering Data | Links | Events | For Sale | Gallery | Books | Videos | About Us