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XPAG and Y Type Threads and Torque Loading.


To the uninitiated a thread is a thread. But if you take a few minutes to think about it, a thread is actually a coiled-up taper, a long wedge. The angles of the thread are shallow enough to actually ‘lock’ together (a bit like a Morse taper as used in engineering workshops). There are big thick, coarse threads and very thin, fine threads. Again, a very close examination will show that many of them actually have a very similar ‘form’, that is the pyramid shape of the thread cut into the metal. It is often 55 degrees and this is not by a mere chance so read on.


When the industrial revolution started, each individual company developed its own thread system. This was as true in the UK as it was in Europe. So eventually you had thousands of different thread types around, and very little was interchangeable. In the UK we used inches & feet and in France they used millimeters and metres, so we had Imperial sized threads and Metric sized threads. The bolts, nuts and set screws all had different heads, some were square (Rolls Royce for instance) and some were hexagonal (ie, Morris).


Prior to World War One, a chap called Whitworth had standardised our UK system and his threads were used on mostly agricultural equipment and marine use; this thread is called British Standard Whitworth, or BSW. But the Whitworth thread is very coarse and vibrated loose when things began to spin round quickly very easily, so a ‘finer’ version was also used called the British Standard Fine thread, BSF for short.


As this had more threads per inch (tpi) it was less prone to undo itself and was used on steam and internal combustion engines,etc. For small things like watches, we had the British Association threads (oddly actually based on millimeters so is Metric), called BA and very fine indeed. For cycles and motorcycles there was the British Standard Cycle Thread BSC, ; gas pipes had British Standard Pipe threads or BSP (actually a version of BSW) and sometimes called the Brass Thread. So by the time the first cars were about in the UK we had BSW, BSF, BA, BSP and BSC. Add to this the cars we imported using Metric threads, and the system becomes a nightmare. Interestingly the thread ‘form’ used by nearly all these systems copy that of Whitworth, 55 degrees. This is the strongest and best ‘locking’.


Then WW1 arrived and quite a few firms in France relocated to the UK to get away from the damage. One firm was ‘Hotchkiss Et Cie’, an armament company who made guns. Morris found them work after WW1 making the engines for his cars, but as the company had its own thread system, an early METRIC system, the XPAG engine has very early METRIC threads. So that the average garage mechanic in the UK spanners would fit, they put Whitworth sized heads on the nuts, bolts and set screws.


So to confuse us today we find our BSF and Whitworth spanners and sockets fit the hexagonal heads on the XPAG engine, but the threads are not either. Other MG’s that use these Hotchkiss metric threads are the TA, TB, TC, TD, TF, VA, SA, WA, YA, YT, YB. The equivalent Morris and Wolseley engine obviously use them as well, (Morris Ten/4, Ten Series ‘M’, Twelve and Sixteen, Eight Series E, Wolseley Twelve, Eight and Ten/4 to mention just a few.)


Then WW2 arrived and we imported lots of equipment from the USA. Now they also had their own thread systems, and it has to be said they were a lot more logical than our hotch-potch of sizes. In the USA they measured their spanner by the size of the nut, bolt and set screw’s head. It had nothing what so ever to do with the thread on that bolt, etc. USA spanners were A/F, that is they were the size Across the Flats of the hexagonal head. On the actual bolts,etc, they had two thread systems, American National Fine and American National Coarse, ANF and ANC, being equivalent to (but not interchangeable with) our BSF and Whitworth. To add complication, we in the UK gave these threads our own names, we called the ANF, UNF; and ANC, UNC. UNF is Unified National Fine; UNC is Unified National Coarse. NOTE:- this USA system uses the bolt head size in its spanners, not the thread size.


All the other systems use the thread diameter. Whilst this was all going on Europe began to sort out its many Metric systems and now has just one, ISO Metric , which again comes in a fine and coarse variety. Alas, only two of the threads in your XPAG engine are in this new Metric system! (Well nearly all; the pipes for the oil pump and filter are BSP).


Unified threads identification markings.During the manufacture of the Y Type, BMC was being formed, so whilst the engine is Olde French Metric, the YA and YT bodies are BSF, during the production of the YB many threads became UNF. For instance the later Nuffield ‘Hypoid’ rear axle is all UNF as are the wheel nuts. A diagram of how to identify UNF threads is shown. Loose nuts and bolts holding things like exhausts on can be anything the last owner found in his box of useful bits. When Austin fully took over Morris, calling it the British Motor Corporation ‘BMC’, all threads then became UNF. Austin had been using this system since before WW2.


So now you have some idea why your YA, YT and TB’s body shell has BSF threads; your XPAG engine and gearbox have early Metric threads; your YA and YT rear axle and wheel studs are BSF threads and your YB rear axle and wheel studs are UNF. Inside your speedometer and in the cars electrical system you have BA threads. The YA and YT brake pipes are BSF and late YB are UNF. Clear now? Luckily Roger Furneaux supplies the correct old metric threaded items for the XPAG under the name of ‘Mad Metrics’ 01932 243534 and the MG Octagon Car Club also keep stocks.


Now we have all these different threads all over the car we need to look at those most vulnerable to vibrating loose. There is a chart where one can work out the correct amount of ‘tightness’ that will not only ‘lock’ a thread tight but will also hold the thing together correctly. This is called the ‘torque setting‘ as we measure it by the amount of power we need to twist the bolt,nut or set screw’s head. Torque means ‘turning motion’. On an old engine like ours it is measured in pounds-feet; that is the number of pounds force needed to a one foot long spanner. Again, luckily we have things called torque wrench spanners to assist here. Most of these have a gauge on them that gives the reading of the power you are applying to the nut, etc.


Applying Torque setting with a Torque Wrench.Torque Wrench Settings:-

Cylinder head stud nuts :   5.7kg/m;   50lb/f

Connecting rod big end bolts, then on to next split pin hole :   3.7kg/m;   27lb/f

Main bearing cap nuts, then on to next split pin hole :   8.6kg/m;   63lb/f

Rocker shaft 10mm bracket nuts :  42 lb/f

Rocker shaft 8mm bracket nuts :   27 lb/f

Flywheel bolts :   50 lb/f

Small end, ( gudgeon pin ) :  33 lb/f

Timing chest cover :   20 lb/f

Clutch cover bolts :  33 lb/f

Sump bolts :   27 lb/f

Manifold clamping nuts :  20 lb/f

Oil pump stud nuts :   12 lb/f


Where known the equivalent metric reading is given. What you are doing is stretching the bolt or stud’s shank a certain amount so as to get the most efficient ‘lock’ on those threads. In really bad areas of vibration nuts are wire-locked together (as inside the gearbox on the selector shafts) or split-pinned (as on the big ends and main bearing caps). Less vulnerable areas use spring washers.


Cleaning a bolt thread using a die nut.Tapping a thread.Beware the threaded holes in aluminium alloy, these should be a coarse thread as the alloy is quite soft, but on the XPAG the metric threads are used everywhere and very easily stripped. If you do strip a thread, it can be repaired using a ‘helicoil’ set. You re-cut the thread bigger, screw in a coil and lock it in the hole with ‘locklite’ and then you can re-use the original bolt.


On ‘posh’ car engines such as Rolls Royce, they do not use torque wrenches, they measure the length of the big end bolt first with a micrometer, then tighten the nuts up until the bolt is stretched a certain amount. It is the same thing really, but be aware that you get different torques on ‘dry’ and ‘oiled’ threads. All the above in the XPAG list are for ‘oiled threads’.


Thread gauge.You can buy ‘thread-gauges’ from any decent tool shop, and this will help you identify those threads you can find no data about. Threads are classified by their diameter and numbers of threads per inch , tpi. For instance a 3/8” diameter brake pipe union (as fitted to the YB system) has 26tpi. To clean up threads or cut new ones we use things called ‘Taps and Dies’. It is worth having one of the more common sizes used on you car handy. For XPAG threads you will need to contact MadMetric.



Now a little test to see if you were paying attention, write down the full names of the following:-




(The SWG was a wind-up. It actually stands for Standard Wire Gauge as is how wire and sheet steel thicknesses are

measured! Your cars body is mostly made of 22SGW, cold-rolled, mild steel sheet.)


XPAG Threads:-


XPAG Threads.
Thread. Tapping Drill. Equivalent. Location. Spanner Size.
5mmx0.8mm 4.2mm ISO metric fan blades 8mm or 1/16” A/F
6mmx1.0 5.0 ISO metric oil pump bolts 1/4” Whit
Tappet chest cover 5/16”Whit
8mmx1.0 7.0 ISO metric sump, gearbox, rocker shaft, bell-housing, water pump, Morris back axle, manifold, timing cover, small end, clutch. 1/4” Whit
10mmx1.5 8.5 ISO coarse Cylinder head, rocker shaft. 3/8” Whit
12mmx1.5 10.5 unique main bearings 9/16” Whit
28tpi 8.7 1/8” BSP Oil pipe to head 7/16” Whit
19tpi 11.7 1/4” BSP Oil filter banjo bolts 9/16” Whit.
10mmx1.5 8.5 ISO coarse Brass plug in pump 1/4” Whit
12mmx1.5 10.5 unique oil gallery screws screwdriver.


To help you with your spanners, a list of shank diameters with the relevant spanner size follows;-

Spanner Jaw Sizes.
Bolt Shank Diameter Spanner Jaw Size Inches
6mm 10mm 0.4”
7mm 12mm 0.478”
8mm 14mm 0.557”
10mm 17mm 0.675”
12mm 19mm 0.754”
BA. Bolt shank diameter Spanner Jaw size
2BA 0.185” 0.328”
0BA 0.236” 0.418”
BSF & BSW. Spanner Jaw size
3/16BSF & 1/8BSW 0.338”
1/4BSF & 3/16BSW 0.450”
5/16BSF & 1/4BSW 0.531”
3/8BSF & 5/16BSW 0.606”
7/16BSF & 3/8BSW 0.718”
1/2BSF & 7/16BSW 0.828”
9/16BSF & 1/2BSW 0.929”
UNC,UNF,ANC & ANF ( A/F sizes) Spanner Jaw size
3/16” 3/8”
1/4” 7/16”
5/16” 1/2”
3/8” 9/16”
7/16” 5/8”
1/2” 3/4”


Using an adjustable spanner is a lazy way to ruin the corners of the flats. An open-ended spanner is better, but the best methods are ring spanners or sockets. Sometimes even the old box-spanners may be useful, (as on the engine steady bar under the water pump.)



Neil Cairns.