73,65mm bore block??

[Vegard]

I just measured what I thought was a 1380 block, but the block is very consistent and parallell and measures to 73.65mm
I've never had the pistons from this, and the block is ooooold.

Was this a common oversize back in the day? Mark??

[guru_1071]

v

i had something similar recently

it had been rough bored to 73.5, then honed bigger.

it was junk

[Smiffy]

It'll be one bored for fiesta pistons.........some dodgy company down south was using 'em.

[36inter]

73.65 was for Triumph Spitfire pistons, one of Vizards options in the 70's, a poor mans big bore engine, it actually makes the engine 1385. I have one in the garage, built in the late 70's and used in my grass track racer for three years. It put out 90 bhp at the wheels @ 6000rpm on Aldons rolling road. Pistons needed shortening but after three years racing they are still in perfet condition.

If I can dig out the receipt from Aldons I will post it, interesting old document.

Pete

[guru_1071]

It'll be one bored for fiesta pistons.........some dodgy company down south was using 'em.

not that far south, you only have to go to the hippercrudderfax* area of yorkshire to get engines of that sort of quality


* this is an area of yorkshire that contains towns such as huddersfield, wakefield, leeds, bradford, halifax etc etc (but not harrogate, cos we are posh)

[Vegard]

So you're saying that the Triumph Spitfire pistons have got the same wrist pin diameter and possibly the same gudgeon pin height?
If so, I guess its easy to find some high performance Spitfire pistons?
Cheaper than having some Wisecos made?

[36inter]

Vizard did a lot of articles in Car & Car Conversion Magazine in the 70's and one of them was about using Triumph Spitfire pistons, I think I have the article somewhere so I will have a look for it, I had a quick look in his book but cannot find a reference to it, as he said at the time it was a poor mans way to a big bore motor, if I find it I will post it up. From memory they are a direct fit apart from shortening the skirt by about half an inch or so, long while ago now. I think there was two types of piston, one high compression and one low compression but I cannot remember which was the one to use, as I said I will try and find the article and post it so we have a proper answer.

I have posted the receipt from Aldons when I had it on the rolling road in 1979. The head was a 163 modded by me in the proverbial garden shed, 544 cam and 45 Webber and it pulled like a train, I have posted a video on YouTube of the engine running, this was about 5 years ago when I put it back together after it had been sat in the airing cupboard in bits for 20 years. It ticks over like a tractor, I love it.

Here is the link to the video :- http://youtu.be/2QdArFOT3Q4

[36inter]

I have just found the receipt for the pistons, the part number is either 15463 or 18463 and they were very expensive in 1977, £9.86 each!!

A little more info on the engine, I ran a 4 syncro close ratio straight cut box and straight cut drop gears.

Pete

[ianh1968]

I too, vaguely remember a reference to this size...

It may have been in the Vizard's paperback book "How to Modify Your Mini", which came out before the "A-Series" engine books. I have the book in my loft, somewhere, but it's too cold in there to go ferreting about for it at the moment. (Northern reference)

Were these 73.65mm pistons made by "Mahle"?

Don't forget, in those days the average tuner could only use pistons that were available as standard in other engines, whereas now they make Mini-specific pistons in many sizes which were not previously available.

Again, did they originally use Simca pistons for 74mm?
They did use Triumph 2.5 pistons for 74.7mm/1425cc on a standard crank.

So far as fittment was concerned, if the pin was the same size, and the deck height was not too far out, anything went, even if the pistons were not really suitable for performance use!

The favourite small-bore conversion was the "Hilman Imp/1098" conversion giving 1216cc as standard and obviously more in Imp oversizes. These pistons did fit OK, and I have had an engine like this myself in the past. The problems with this setup were that even with the 1098 crank, you had to mill a large chunk off the top of the block to get them near to the top. The thickness between the top of the piston and the first ring was also quite a bit less and the skirts were shorter.

Although we are talking about small-bore here and the original post was big-bore, these points are still relevant. Shorter pistons rock about more and wear the bore oval faster and the thinner deck/top is much more likely to pinch the top ring if the mixture is a bit lean and the engine runs too hot and/or detonates. Erm, I know all about that - still got the pistons as a souvenir.

The block certainly would not be scrap at 73.65mm, 74mm would be OK and 74.7mm a bit on the thin side, just be wary of the other parameters mentioned above.

[36inter]

Ian, you nudged the memory, brilliant, thanks. David Vizards 1977 Book "How to modify your Mini" is the one, just got it off the book shelf, this is probably going back a bit far for some of the young-uns on here

The Mahle's were the 73.5 pistons. I have copied two pages, one shows a chart of what was available then with regard to pistons and the first column of the other page has some details about the Spitfire pistons and their fitment. The bottom of the chart gives piston number details, the Spitfire number being 18463 not 15463.

The offsets for the bores was printed incorrectly at the time and showed all bores offsetting to the left (which I followed when boring my engine, luckily it all worked ok) I have redrawn it to show that the bores, in pairs, should move away from the centre of the block.

Vizard also wrote a book called "Theory and Practice of Cylinder Head Modification" first published in 1971 and this is the book I followed when modifying my head, it is well thumbed with quite a few of the pages falling out, its an excellent book although I expect head modification practices have come a long way since 1971. I have posted a copy of the cover, the title writing is in yellow and does not show up well on a scan.







Hope its of interest.

Pete