mk1-forum.net

Ok this last week i have been thinking up a plan to make a pretty potent sub 1000cc race car with crossflow head. Initialy i was thinking of going to town on an 850 block because it is basically a short stroke 998. Questioning the strength and reliability at 9000rpm and 100+hp im going to knock that on the head. While doing some simple maths however, I realised that if you make the 850 square it gives you near as damn it 999cc. It then struck me that if i use a 1071 bottom end (which i can currently get hold of) and fit liners to reduce the piston size to 68.25mm I will have a good strong bottom end yet still be under my 1000cc maximum. Is this something they did back in the days of the 1000cc classes? Seems funny how it works out at what i want nearlly perfectly square. In my case it could be a happy medium over the 999 using the 970 crank as itll likely give a little more torque yet still a good rpm range.

The "usual way to make a short stroke 1000 is to over bore a 970 S to +40 which also gives 999cc.

Your way is possible, but you would be a bit limited on piston options, unless you get some specials made.

M

Yes it took me a while to find the correct pistons but i have found a small sellection originaly designed for 16v layouts. I did see that CalverST sell some competition piston at this size but were for highly over borrd 998 blocks. Not sure how suitable they will be?

Using a 1071 bottom end would seem an odd choice when you could just use 970 crank , rods and a cheap 1275 block machined to suit.

You'll find this an interesting read :

viewtopic.php?f=3&t=6064

Pete wrote:Using a 1071 bottom end would seem an odd choice when you could just use 970 crank , rods and a cheap 1275 block machined to suit.

You'll find this an interesting read :

viewtopic.php?f=3&t=6064

Ideally i would Pete but finding 970 stuff is the problem though and especially at a realistic price. I also think using the 1071 crank will give me a little bit more torque. It's all ideas at the moment. I just need a way of achieving 9-9500rpm safely to make the most of the twin cam set up.

Something else to consider is by going to a smaller bore size reduces the size of the valves you can fit and increases valve shrouding. Big bore, short stroke is a better combination, also reduces stresses in the bottom end, more so at higher RPMs.

I agree with spider about valve de-shrouding, but I guess you're thinking the 1071 route because you can access a crank, but this really all comes down to optimising rod to stroke ratio, along with suitable cam timing, there are plenty of papers written on the subject!

It's important to consider everything about the build, I have a friend in the US who has a MG Midget he runs at Bonneville in a sub 1000cc class, he went for a 970S stroke crank and custom length rods, but his choices have given him a lot of grief later in the build, getting enough compression ratio (for a race engine) and having the ultimate cam lift / profile can come down to thousands of an inch clearance on these types of engine.

Chris Conrad (Milwaukee Midget) has a build diary: http://www.landracing.com/forum/index.p ... 087.0.html

It's a huge build thread, but if you have time, try to read some of the posts by (Mark Balinski) 'Ford Boy' he has kind of adopted the project and he and Chris have really got the Midget screaming!

To give you some idea about the technical input, Mark has just been doing some interesting flowbench studies on cylinder heads including Swiftune items! http://www.landracing.com/forum/index.p ... .3555.html

Thanks

graham in aus wrote:
It's a huge build thread

Well, I think that rather understates it,,,,,,, 239 pages,,,,,,, and still going.

I think I got as far as about page 78 - ish, only after skipping the first 40 or so pages!

BTW, the thread in the nose of the crank from factory is 5/8 16 TPI, it's a British Small Arms Thread. Taps and Does aren't that hard or expensive to come by.

The few pages that I did look at were rather interesting. And yes, I'd very much agree with DV, get that CR up up up! and bigger LCA's.

I do rather like Fordboy's attitude / approach. Rather refreshing, as unlike so many these days, he actually keeps an open mind and will only run with proven facts.

Couldn't you acquire a SA 1100 crank and destroke it? but then i don't know if you could use normal S rods (as they may be too long)

i was thinking the same.

De-stroke it by 3mm.

using 73mm pistons though, its still an 1100.

Std 70.64 bore on a 63mm stroke its 1035cc.

How would you destroke it, you would then need to have con rods made to suit the smaller journal crank. It's not like you could use S rods on it.

Smiffy wrote:How would you destroke it, you would then need to have con rods made to suit the smaller journal crank. It's not like you could use S rods on it.

you can destroke a sa1100 crank to use the s rods as they use the larger rods as standard.

I don't think you can move them by 3mm though..................


its common in auto grass to stroke them for the s rods as this lets you get them bang on the class limit of 1130cc

It's also how you can get 1430 or 1453 or what ever the biggest size is. But must admit I didn't know SA1100 used the larger rods.

Smiffy wrote: But must admit I didn't know SA1100 used the larger rods.

stick with me son, i'll help you out

Whats an SA 1100 if you dont mind me asking? Ive never heard of one before now.

As a heads up, ive just been offered a few 970s bits from a chap i met through the clio v6 owners club. He has a fully built up Coldwell engine with normal 5 port head, an oem 970 crank with +10 on each bearing and a small selection of 970/1071 blanks which were forged back in the 60's. He seems a top bloke so im hoping this might help out a fair bit.

rich@minispares.com wrote:

Smiffy wrote:How would you destroke it, you would then need to have con rods made to suit the smaller journal crank. It's not like you could use S rods on it.

you can destroke a sa1100 crank to use the s rods as they use the larger rods as standard.

I don't think you can move them by 3mm though..................


its common in auto grass to stroke them for the s rods as this lets you get them bang on the class limit of 1130cc

If you can get a 81,33mm cranskshaft offset reground to 84mm, then you are almost on 3mm

As sold by MED.

Why not sell the 1071 crank and byu a 970. That's the way to go.

Vegard wrote:If you can get a 81,33mm cranskshaft offset reground to 84mm, then you are almost on 3mm

As sold by MED.

Why not sell the 1071 crank and byu a 970. That's the way to go.

You can get a unground crank ground to about 85mm.

Oldskoolbaby wrote:Whats an SA 1100 if you dont mind me asking? Ive never heard of one before now.

.

it was a crank that was only ever made in south Africa as some sort of tax wheeze as it converted a bog standard 1275 engine into a 1100cc by using one of these cranks and some stupid pistons with really thick tops.

as a result it was a useless slow revving engine that was ignored by most people - everyone in SA just wanted a 1275 crank and some decent pistons to convert them into a proper 1275cc engine.


if, however you use one of these cranks and deck the block, then you have a '1071' cc engine at the fraction of the cost of a proper cooper s one.

the cranks, however are bomb proof - even though they are 'only' EN16 the journals are so large that they are very strong. ive never, ever heard of one breaking or causing problems. even with some of the epic blow ups and over revs my race car has suffered from over the years, the crank has never been damaged or harmed


ive used for years, both as standard 'off the shelf' cranks and fully worked race specials

im a big fan of them and 1071 engines as a result

Hello Oldskoolbaby

I would be interested in the un-machined cranks if you buy those bits .

Thanks

Simon , orx77@yahoo.co.uk

Mark, Rich,

This forum is wonderful. Now I'll waste untold hours this weekend researching "SA1100 crankshaft"...

cw