4 replies to this topic

[richardjackson99]

Has anyone any info/experience of this, please? Sorry if I'm duplicating anything but I couldn't readily find a thread on the subject.

To explain:
I have an "A+" block which is currently a 1330, however it's going in for a rebore, hopefully to 73.5, but maybe 74mm (depends what it takes to remove the pitting), or maybe it'll be for the bin! We wait and see what comes back!! Reading Vizard (page 304) he says these blocks will "stand overboring to at least 74.7mm, so I'm hopeful.
I see from the "How big is my engine?" page that both 73.5 and 74 bores can be used with an 84mm stroke. On this, Vizard says you should beware strokes of 86mm and above. So I'm interested in whether anyone on here has experience of an 84mm stroked crank, which would give 1426 or 1445cc, depending on the bore.
If anyone has any knowledge on this, I'd appreciate it - including a suggested supplier/engineering company.

I did read the article in Feb 13's MiniWorld, and I then contacted MED, but got no further forward.

Many thanks.


Richard.

[Cooperman]

First of all, to bore beyond 1330 cc, i.e. +0.060", it is vital to offset bore to give sufficient 'land' between cylinders 2 & 3.
Of course, at over 73.5 mm there is always the risk of scrapping the block as the manufacturing tolerances mean blocks can vary quite a lot.
Stroked cranks are achieved by grinding the big-end journals offset further out from centre line and to Cooper 'S' size of 1.625" diameter nominal down from 1.75" nominal.

[MRA]

Its one oil way that will potentially scrap the block and even then these can be repaired, but its simple to check to see if the block is thick enough in this area, ie the oil way has been drilled away from the cylinder.

There are 2 types of stroked crank for the Mini, one is as outlined by Peter above...

The other is a new forging or billet, these are still available if you know where to look, however they are not cheap.

The reason why 86mm is as far as you want to go with an A+ block is because the "engineers" did a lot of work onmaking the engine more reliable, some of this was done by "sculpting" the block to suit its application, this meant that it could no longer be ground out to fit the longer stroke crankshaft, if you use an earlier A series block you can stroke it up to 92mm, well this is the longest Ive seen in an engine, coupled with 74.7mm pistons gave about 1630cc excellent rallycross car.

[crock]

Since the connecting rod's big end journals are ground down to 1.625" you will need Cooper S style connecting rods with the matching big end diameter. Then you have to get your piston's deck height matched to your block. In the USA we machine custom pistons for the correct deck height, in the UK you can buy 73.5 pistons with the correct (shorter) deck height from MED and Swiftune. After a trial build, you can measure your deck height and get the head or block skimmed for the compression ratio of your choice.

[MRA]

Early Midget rods will also fit, and are pretty good, just not quite as good as the Cooper S