5 replies to this topic

[Milwaukee Midget]

As an MG guy, I'm a bit out of my depth here, but I suspect that this is the one forum that may be able to help me.

I'm building a 1971 MG Midget for a race class that limits the capacity of the engine to 1 litre. While the 948 would be the logical starting point, the 1275 block lets me run a shorter stroke and a head with bigger valves.

I'm hoping to discuss converting the 1275 inline block to 970 specifications.

I'm aware that the crank will need to be a custom piece. I'm also aware that the connecting rods are somewhat longer on the 970, and that the 970 Mini block is somewhat shorter than the standard 1275 deck height, but I don't know by how much.

I'd like to correspond with someone who is knowledgeable on the 970, and I'd be grateful for any links that might be helpful regarding this unusual "A" series motor, and my rather unconventional engine build.

Thank you for your time.

Sincerely,

Chris

[Vegard]

What do you want to know? It's just the stroke that's different. And the obvious lack of torque You seriously need to rev the socks of it to get going.

If you're having a crank made, why not get one made with 1.75" big end journals for extra strength and rigidity? The Cooper S was 1.625"

[Guess-Works.com]

essentially the block will remain the same, but if you wanted to use an A+ block look for one out of a Montego or Maestro ( or Ital ) I believe they are the same as the inline block, with the difference being the front main cap being different to the transverse engine.

I'm going through a similar process building a short stroke mini engine based around the 1071 crank, using an A+ block and will be boring this to 73.5mm as this will help the lower end of the torque curve. To help with the upper rev range the block has had the caps replaced for steel caps and the centre is now a 4 bolt steel cap.

While typing this I've been mentally adding up the potential cost of the engine, when considering the crank, rods, pistons and other machining just getting the short engine upto spec, my immediate thought is, are you limited to using the A series engine... I have a feeling that a bike engine would turn out to be a more economic and probably better solution ??

[JetBLICK]

The 970 s engine has a stroke of 62mm compared to 68 of a 1071 and 81 of a 1275. All of the engines as you've probly found out by now have the same 'big bore' of a bit over 70mm iirc.

The 1071 and 970 and 1275 all shared the same block, however for the 1071 and 970 3/8 of an inch was shaved off the top to compensate for the stroke. The 970 also had longer con-rods.

Altho the 970 is a motor where you'd have to rev the nuts off it, i'd imagine, compared to a 948, it'd be infinatly better especially for racing. Your only issue really would be getting the crank made. As the in-line a-series cranks are very different to the mini cranks. Having said that, if you where to get a crank made with a 970 stroke you could also have it made so that it fits straight into your standard (albeit heavily skimmed) MG block which would save a bit.

[Vegard]

As already mentioned, you would normally need to shorten the block in some way.

If you are prepared to fork out for a new crank, why not have a set of "long" rods made as well?

I know you can buy a kit from here:

http://www.specialis...gine Components

The A series engine do have a fairly long rod/stroke ratio, but longer rods will help hp and combustion. For a proper racing engine build, use the block as it is ( easier at a later date) buy crank and longer rods to make it all fit. Obviously you cannot use a Mini block, but a Midget or Marina would be easily to find.

Have a look at my 1071 *illegitimate person* engine build here:

http://home.online.n...f...=&pExPic=0;

[62S]

Some 1960's BMC Formula Junior engines used an inline 970 - I'm sure there are some raced in historics somewhere so they may be able to help with a crank.