Original type S exhaust manifold

[Andrew1967]

Before I get a quote to get my manifold ceramic coated, can you confirm if this is an S manifold rather than a 998 ??

It does fit but is very close to diff housing but not touching (Rizla clearance). Rubber coupling is fine.

Cheers

[Andrew1967]

Have now tweaked the manifold at the bottom and got about 3mm clearance at tightest point.

[Nick W]

Couldn't tell you if it's an S one or if theres a difference.
But when I've fitted a maniflow one , it was just as close from what I remember.
Also check with the remote housing on as it can be tight where the two pipes join together.

Nick

[Lakeland997]

If you bolt the exhaust clamp to the boss on the remote then the manifold will move entirely with the engine and clearance to the diff housing won’t matter.

Andy

[Peter Laidler]

Yep, same here. I agree with Lakeland.

Can I make a suggestion Andy? With your welding skills, what about putting a new pipe end onto the manifold? And while it's off and looking sound but a bit fragile, bend a strip of 3/8"-12" wide 1/8" thick strip of steel down/along the curvature where the manifold turns to the rear. Did it to mine after the local BMC/BL/Unipart garage man here told me that splitting down here was the principal reason for warranty complaints. More to it than that of course but any reinforcement make sense to me.

[goff]

(Rizla clearance)

A lot of folk won't know that means Andrew

[Peter Laidler]

As apprentices in the 60's, we called a clearance like that ' a gnats knacker'. In Australia, where political correctness - or ANY correctness - didn't enter the Australian Army vocabulary of the day, a gnats knacker became a RCH. I didn't understand it, being young and simple. Maybe one of the antipodeans can enlighten the poms what a RCH is!

[IslandBlue]

Was there any difference in the height of a 998 vs S manifold to allow for the difference in the height of the block?

Or was that a) not worth worrying about and b) a figment of my imagination?

[Andrew1967]

Was there any difference in the height of a 998 vs S manifold to allow for the difference in the height of the block?

Or was that a) not worth worrying about and b) a figment of my imagination?

Thats what I was wondering too. I'm sure I've read it somewhere that there is a difference but then again that doesn't always mean a lot

I'll have a look in AKD3509, see if that says anything ..

[Andrew1967]

(Rizla clearance)

A lot of folk won't know that means Andrew

I was assuming that most of the regulars on here were old enough to know Goff

Dangerous word 'assume'

[bwaminispeed]

I believe the S one is taller, and, the subframe is the clearance problem, not the diff.......

However, as, we all know, I may be wrong.....

[Andrew1967]

Had a look in AKD3509 and there are two, possibly three (or its a typo) numbers.

The 1071 used 12A271 as did the 997 and 998. The 970 used 12A721 (typo ??) but then used 12G615 and the 1275 used 12A271 and then 12G615.

So from that, it seems like the manifolds were all the same and part number may have changed when the 970/1275 were produced.

But, as we know the parts books are not always correct

Anyone able to set the record straight. Here is a pic of my manifold with a tape rule from top to bottom - approx 22"

[winabbey]

Here's my contribution to the discussion, for what it's worth.

According to the BMC engineering drawing the 12A271 Exhaust Manifold was approved for release into production on 11 Jan 1961. While adjustments were made to some of the dimensions in Mar 61, Sept 61, Mar 63 and June 64 the part number remained the same. The changes were minor variations and clarification of dimensions, mainly around the mounting flanges. No change to the length or profile.

The 12G615 Exhaust Manifold was approved for release into production on 6 Mar 1965. Two subsequent changes were made to the design but these are indistinct on the drawing. It appears to be a straight copy of the 12A271 drawing, except the vertical distance from the centre of the pipe at the mounting flange to the centre of the pipe where it exits is 18 29/32" versus 18 5/8" for the 12A271 (about a quarter of inch).

So a 12A271 seems to be virtually identical to a 12G615.

The BMC Australia parts book shows 12A271 as the manifold for the Australian built Cooper S with 12G615 an alternative, reinforcing the view that they are the same, or the difference is so small as to be inconsequential.

While the drawing shows a weld where the three pipes connect to a single pipe half way down I have two examples of manifolds where the there is no weld. In other words, they comprise a front section and a rear section that, when welded along their sides, form a complete manifold from mounting flanges to the short pipe welded on the end where the main exhaust pipe connects. The pic below is of one set taken just before being handed over for ceramic coating.

Extractors Ceramic Coating.jpg

[Andrew1967]

That's excellent Winabbey, thanks for posting that.

With that approval date its possible that HOY, built late March 65 could have had either manifold.

I'll see if I can get a measurement as mentioned and also see if that weld is present.

[Peter Laidler]

I mentioned this then showed the thread to my neighbour Terry an ex high-up at Unipart-Cowley for years and years (or tears and tears as he says....). His first words were '......yes, too costly to make in-house so it was being prepared for contracts until a BMC designer came up with a new idea for an in-house pressing.'

Quite WHAT the new idea was he doesn't recall. But after looking at the thread photos and parts list drawings - and my car, we came up with a feasible and economic possible answer..... Was the original (and part number ?) a tubular welded design that was too expensive to produce in house. Until the two piece pressed design as shown, designed IN-HOUSE came on stream. Cheaper, stronger (?) easier to produce etc etc. It would be early to mid 60's he says but definitely low volume. He's a mine of info and is rarely wrong about spare parts production.

[winabbey]

This is a sub-set on the 12A271 drawing showing the component parts and their part number. I'm guessing the Front, Centre and Rear description is related to the front of the engine (i.e. radiator end) as if for a north-south configuration.

Nothing on either drawing mentions welding the entire length as would be required for the common pressed metal manifold shown in the pics in this thread.

12A0271 Exhaust Manifold Components.jpg

[iain1967s]

Quite WHAT the new idea was he doesn't recall.

Some useful photos on the prior thread here. http://mk1-forum.net/viewtopic.php?f=3&t=18120

In particular, the later (black in photo below) pressed steel manifold appears to have the seams continuing farther down than the earlier version. i.e. where it changes from pressed to welded tube is higher up on the earlier parts.

So maybe the cost reduction is that they only needed to weld on a short stub pipe at bottom, rather than forming the compound curve on the longer down-pipe.

[Andrew1967]

Thanks Iain,

From that, it seems that mine is the later type then.

Interesting post Peter, and I'm sure it must have been down to ease of production or cost effectiveness.

[iain1967s]

I think the later type are probably stronger too, with those seams running the whole length both sides.
This is what happens to the earlier welded type when the engine starts to rock on worn mounts, stabilizer etc.

[winabbey]

Just noticed in the list of component parts on the 12G615 manifold drawing that the exhaust pipe sleeve has a different part number to the 12A271, with all the other components being the same as for 12A271. I don't know what the difference would be - length, diameter, ??

12G0615 Exhaust Manifold Subset.jpg