Engine bore centres

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gazza82
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Engine bore centres

Post by gazza82 »

This may sound an odd (or daft) question but is the centre of the bore the same distance on all engine sizes? :?:
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Re: Engine bore centres

Post by Dave Clark »

Can,t find the bore centre distances at the moment, but it is inconceivable that the 803, with a bore of 58mm could share the same bore centre with a 1275 with a bore of 70.62 mm.

Even 950 to 1275 ( 62.9mm to 70.62mm) is imposssible.

I know that 1098,s have been stretched to 1251cc , using Hillman Imp pistons) but the problem with large A series overbores has always been sealing the bores between No 2 and No 3 cylinders.

Big overbores on 1275,s are often achieved by offsetting the bores to each other to gain a little more "meat" between 2 and 3.

I,m sure you know that there is a basic difference between the "small bore" A series blocks (803/948/998/1098 and the "big bore" ones (970/1275)

Why do you ask ?
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Re: Engine bore centres

Post by gazza82 »

A question someone asked me. Trying to find out why they want to know but triggered my inquisitive side. I will look through my various manuals later.
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Re: Engine bore centres

Post by Big Daddy »

Dave Clark wrote:Can,t find the bore centre distances at the moment, but it is inconceivable that the 803, with a bore of 58mm could share the same bore centre with a 1275 with a bore of 70.62 mm.
I couldn't find any data on A-seies bore centres either... :cry:

But I have a suspicion that bore centres are common across all A-series engine capacities, partially backed up by experience of fitting a Marina 12G940 head to 948 block. It's common industry practice for each engine family (eg A-Series, B-Series or Kent) to maintain common bore centres.
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Re: Engine bore centres

Post by Dave Clark »

Maybe small bores have the same centres and the large bore ditto, but I still cant see them all having common centres.

12G940 to 948 requires block pocketing as the valves overlap the bore and may also be offset - not sure about that though. Offering a 1275 head gasket up to a 948 block could be illuminating.
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Re: Engine bore centres

Post by Nelly »

Found this thread from another forum which may give a clue (second post).
http://retrorides.proboards.com/thread/ ... re-spacing
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Re: Engine bore centres

Post by gazza82 »

Which says:-
- 998cc A-Series (64.58mm bore): cyl.1&2/3&4: 5.5mm, 2&3: 18mm
- 1275cc A-Series (70.66mm bore): 6.5mm

The smal-bore engines (848, 948, 998, 1098) have paired bores, hence a bigger spacing between 2 and 3. The big bore engines have even spacing (970, 1071, 1275). P.e. 848cc vs. 1275cc:
Many thanks for the link!
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Re: Engine bore centres

Post by David Swain »

I have a Mini 850 cylinder head, on my Ex 948, now 1030 Engine. All seems to line up OK :D
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Re: Engine bore centres

Post by Big Daddy »

Dave Clark wrote:Maybe small bores have the same centres and the large bore ditto, but I still cant see them all having common centres.
You're correct Dave - I should have known better than to debate such issues with an A-series expert :bow:

Photos below clearly show difference in bore centres between small and large A-series. This means different capacity A-series blocks could not go down the same machining lines unless fundamental tooling was changed. That's an expensive penalty and would not be tolerated by any manufacturer focused on making profit. No wonder BMC, BLMC, Austin et al went out of business... :roll:
848 block.jpg
848 block.jpg (188.73 KiB) Viewed 289 times
1275 block.jpg
1275 block.jpg (32.06 KiB) Viewed 289 times
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