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Can anyone advise date or serial number sequence for when Mk1/2 Cooper S thin flange block changed to thick flange?

I’d say very late 69....

Sometime in 1968...?

68 and early 69s were thin flange blocks.

Many thanks for the inputs, much as I thought but could not see this in any of the popular books.

I would say April / May 69 although early versions are rumoured to have be used in works cars before that date .

Some early 1275 thin flange blocks used in racing had centre main bearing failure .

This resulted in the introduction of a thin flange block with strengthened solid centre main bearing at some point 1966.

The works minis book referred to this as works strong block

My 69S built June 6th has thick flange block, engine no. 9F-XEY-536**

Here are a few blocks to help you compare to.

Thin – 9FXEY 51631, 52463, 52973, 53002, 53572, 53574, 53617, 53725
Thick – 9FXEY 53765, 53910, 53911, 54529, 54575, 54665, 54701, 54812, 54896,

I am not sure if they used a mixture of thin and thick flange blocks during the changeover period. If its anything like their other parts used - sometimes older stock gets mixed into the newer stock so a definite change point can have the odd exception.

So all 9FSAY are thin flange?

GTOGREEN1969 wrote: ↑Fri May 14, 2021 8:58 am Here are a few blocks to help you compare to.

Thin – 9FXEY 51631, 52463, 52973, 53002, 53572, 53574, 53617, 53725
Thick – 9FXEY 53765, 53910, 53911, 54529, 54575, 54665, 54701, 54812, 54896

It would be really interesting to know the casting numbers on those S blocks.
It’s usually on the back center area near the fuel pump boss.

My mid August 69 built ( 47000 miles, known history ) has thin flange. 9F-XE-Y/531xx
Hope this helps.

Can the date/period of a 1275 thick flange block with a casting number of A30D9 & AEG 312 be identified by anyone?

30 April 1969

(Can the date/period of a 1275 thick flange block with a casting number of A30D9 & AEG 312 be identified by anyone?
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Re: 1275 Block datesQuote 251 ENG
by 251 ENG » Sun May 16, 2021 11:39 am

30 April 1969

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Thanks for the info.

For a reference point, the above casting number is on a thick flange 1275 block with an engine number 9F-XEY-536** in my early June 69 S

iain1967s wrote: ↑Fri May 14, 2021 4:21 pm

GTOGREEN1969 wrote: ↑Fri May 14, 2021 8:58 am Here are a few blocks to help you compare to.

Thin – 9FXEY 51631, 52463, 52973, 53002, 53572, 53574, 53617, 53725
Thick – 9FXEY 53765, 53910, 53911, 54529, 54575, 54665, 54701, 54812, 54896

It would be really interesting to know the casting numbers on those S blocks.
It’s usually on the back center area near the fuel pump boss.

The following are some numbers I have noted from photos of S blocks. Some photos were a bit blurry and some 6s may be 8s. You may need a larger cross section of engines to get a better understanding. It doubt an engine number could be worked out via the casting numbers as they most certainly would have been jumbled around before the engine number was assigned.

B 8J5 – AEG312 thin flange 9FSAY 3904x
B6A6 – AEG312 thin flange 9FSAY 4162x
A10L7 – AEG312 thin flange 9FSAY 4797x
A23L7 – AEG312 thin flange 9FSAY 4812x
A 7K8 – AEG312 thin flange 9FSAY 5163x
A 4G9 – AEG312 thick flange 9FXEY 5457x
A 2H9 – AEG312 thick flange 9FXEY 5473x
A 9K9 – AEG312 thick flange 9FXEY 5489x
A 5M9 – AEG312 thick flange 9FXEY 5549x
A29A0 – AEG312 thick flange 9FXEY 5577x

GTOGREEN1969 wrote: ↑Mon May 17, 2021 5:44 am

iain1967s wrote: ↑Fri May 14, 2021 4:21 pm

GTOGREEN1969 wrote: ↑Fri May 14, 2021 8:58 am Here are a few blocks to help you compare to.

Thin – 9FXEY 51631, 52463, 52973, 53002, 53572, 53574, 53617, 53725
Thick – 9FXEY 53765, 53910, 53911, 54529, 54575, 54665, 54701, 54812, 54896

It would be really interesting to know the casting numbers on those S blocks.
It’s usually on the back center area near the fuel pump boss.

The following are some numbers I have noted from photos of S blocks. Some photos were a bit blurry and some 6s may be 8s. You may need a larger cross section of engines to get a better understanding. It doubt an engine number could be worked out via the casting numbers as they most certainly would have been jumbled around before the engine number was assigned.

nothing - AEG613 thin flange 9FSAY 3740x
B 8J6 – AEG613 thin flange 9FSAY 3904x
B6A6 – AEG613 thin flange 9FSAY 4162x
A23L7 – AEG613 thin flange 9FSAY 4812x
A 7K8 – AEG613 thin flange 9FSAY 5163x
A 4G9 – AEG613 thick flange 9FXEY 5457x
A 2H9 – AEG613 thick flange 9FXEY 5473x
A 9K9 – AEG613 thick flange 9FXEY 5489x
A 5M9 – AEG613 thick flange 9FXEY 5549x
A29A0 – AEG613 thick flange 9FXEY 5577x

Any one know how the nomenclature works with the casting numbers, they cant just be random

What do the casting numbers AEG 613 and AEG 312 on the block relate to?

Cammsjb wrote: ↑Mon May 17, 2021 9:24 am What do the casting numbers AEG 613 and AEG 312 on the block relate to?

Sorry about that. There is no AEG613. Something happened when I transferred the numbers across from my spread sheet. AEG312 is the S casting number. Later blocks also have AEG634 below the AEG312

I have updated the list above

Exminiman wrote: ↑Mon May 17, 2021 8:52 amAny one know how the nomenclature works with the casting numbers, they cant just be random

From a previous conversation with Paul "TECH396", we believe the date code is cast into the block showing shift/day/month/year - possibly with the letter "I" skipped in the month field but need a bigger sample size to know for sure.

So for example my own two engines B8J5 would be second shift 8th Sept 1965, and A1B7 would be first shift 1st February 1967.


B 8J6 – AEG312 thin flange 9FSAY 3904x - 8 Sept 1966
B6A6 – AEG312 thin flange 9FSAY 4162x - 6 Jan 1966
A10L7 – AEG312 thin flange 9FSAY 4797x - 10 Nov 1967
A23L7 – AEG312 thin flange 9FSAY 4812x - 23 Nov 1967
A 7K8 – AEG312 thin flange 9FSAY 5163x - 7th Oct 1968
A 4G9 – AEG312 thick flange 9FXEY 5457x - 4th July 1969
A 2H9 – AEG312 thick flange 9FXEY 5473x - 2nd Aug 1969
A 9K9 – AEG312 thick flange 9FXEY 5489x - 9th Oct 1969
A 5M9 – AEG312 thick flange 9FXEY 5549x - 5th Dec 1969
A29A0 – AEG312 thick flange 9FXEY 5577x - 29 Jan 1970

Fascinating, so you think the month is denoted by letter from A onwards....this would make sense otherwise if you used first letter, wouldn’t know if it was June or July, for instance.