Double Helical Gearbox

I pulled the engine and gearbox of my '38 SS to investigate and repair the gearbox, which is noisy in all gears but 4th. We got snow today, October 30 is way too early for snow in Chicago, but anyway I brought the gearbox inside to start in on it.
First surprise, why are 2nd and 3rd teeth going opposite directions?
Second surprise, I was expecting the counter shaft to have bushings.
It has needle bearings.

There are a couple of damaged screw heads, and shim stock behind the front roller bearing to move it rearwards a bit. Somebody has been in here before me.
Good news is I found something definite, some wear on the counter shaft from the needle bearings.

Counter shaft measures .9800" all other places, but .9785" at these worn places, worn on the top.
So, did somebody replace the whole box, or just the counter shaft bushings, or some other alternative?
Trying to figure out how I can identify this box. There is a serial number 1549 on the top cover. No prefix or suffix letters, though I suppose there wouldn’t be with a double helical.

No other part numbers that I can find.
A number SGK-80 on the side of the case.

More disassembly and measurements tomorrow.

Hi Rob,

I haven’t got a photo of the side of my box although one side has STANDARD on it. I don’t have numbers around the circular boss on the top cover. My box has bushes rather than needle rollers for the layshaft.
My 2nd and 3rd differ from yours too.

I see that the 3rd gear changes in the coachbuilt cars
later cars…

I was mistaken about the countershaft. There are two versions of the 3rd gear. This is the type of countershaft with separate gears.


Many thanks for those pictures and web site, Peter. My current thinking, subject to correction, is that the 3rd gear set is on backwards, and that could cause noise. My college course in gears didn’t really cover double helicals much as they were already out of style, but I know certain gear types can be noisier than others, so maybe load direction matters with them. I don’t know yet if that is even possible, to put the one on the main shaft on backwards. Certainly the corresponding gear on the counter shaft can be put on the other way. I am on the way out now to hunt down a big snap ring pliers to remove the snap ring that holds that 3rd gear on the main shaft. There are no loop holes in the snap ring, just straight cut ends. Its about 1/8" thick.

Hi Rob,

I don’t think it would be possible for 3rd main shaft gear to be reversed because the dog clutch can only be on the one side and must be fixed to it.

There is definitely a change in the early coachbuilt cars so it may be that at some stage your car has been fitted with the early gearbox.


The number on the top cover is the gearbox number. If it is an original for the car, it should be the same number as is stamped on your data/vin plate. Also if it is an original box, this number might be repeated on one of the square or rectangular bosses on the side of the casing at the top edge, although this is not on a colleague’s box, but is so on my 6 cyl., which have bigger boxes.

I have limited tech knowledge of the engineering principles of a double helical, except for they being devoid of side thrust, as is a straight cut gear. However, I would have thought that the driving gear would turn with the vee pointing ‘forward’, i.e. the peak of the vee contacts the driven gear first and the outer edges last.

Hi Peter,

The data plates on SS and on MkIV cars record chassis number, engine number and body number. Which of these numbers do you expect to find on the gearbox top cover?



No gearbox number was ever recorded at all on the ID plate of 1938 cars, nor is there one on the Heritage Certificate. No numbers at all on any of the 4 rectangular bosses at the corners of the main case. I am familiar with them on SH and JH boxes.
There seem to have been at least 3 gearboxes used on 2-1/2 Litres, but I don’t know how to identify mine, so I thought perhaps others might have seen serial numbers similar to mine. Perhaps such information will lead nowhere, because the parts catalogue gives information on gearbox variations based only on engine numbers.
I sort of had an idea that the synchro dogs and cone were a separate piece from the gear, and could be fitted to either side, but I may be totally wrong there.
I also observe that on Peter’s, 2 out of the 3 gears are going the opposite way from mine. His 3rd/4th synchro sleeve is different from mine.
Didn’t find a snap ring pliers big enough to get that ring off today.

I wonder how much of this debate may be explained by referring to J1, J2 and J3.

These three SPC printed postwar, do actually cover 1938 to 1940MY SS Jaguar, and do fully detail, and illustrate double helical gearboxes.

Without going into detail.

ASS.1 is applicable to 3-1/2 litre cars for Engine Numbers M.1 to M.1500

C.1023 is applicable to M.1501 to M.1600 and S.26 and subs.

ASS.2 is applicable to 2-1/2 litre cars for Engine Numbers L.1 to L.1450

ASS.3 for Engine Numbers L.1451 to L.2055

C.1023 for Engine Numbers 1 to 17 and P.18 and subs.

ASS.4 is applicable to 1-1/2 litre cars, also identified as Part No. 41135, and is applicable to Chassis NOS 50001 to 70688.

50249, or C.1373 is applicable to Chassis NOS. 410001 and subs.

These are all double helical gearboxes, and are fully detailed down to all individual part numbers and indeed line drawings.

If you want detail of superseding single helical gearboxes, then refer to JS.100 spare parts catalogue, or indeed as included in left hand drive J.5 and J.6.


There is another cross section in “The Automobile” article date October 1937. It again shows the layshaft with separate gears but not the needle rollers.


Yes, I’ve had J2 open for several days studying it. My engine is L205.
The presence of needle bearings in the counter shaft leads me to believe this is likely the C1023 last gearbox, as the Ass2 and Ass3 have bushings there.

I got the snap ring off by driving in 4 tapered screwdrivers in the splines, to expand the ring, and then prying the gear itself as a hammer to drive the snap ring off the groove. From there I could remove the screwdrivers and drive the snap ring off the rest of the way using the gear.
Here it is. No needles here, just bushings.

Second and first also slid off easily.

And the conclusion is, no, the synchros are not a separate piece from the gear, it can’t be put together wrong, those double helical gears do indeed go on opposite ways.
Plate J shows the gears in opposite ways, and I was inclined to suspect an error on the part of the draughtsman, but I guess he got it right.

So that seems to eliminate that as a suspect in the source of the noise.
Still not sure why Peter’s are all the same direction and mine are not, but it is simply an earlier model, Ass2 or Ass3.

I found a book with some description of herring-bone or double helical gears, Kent’s Mechanical Engineer’s Pocket Book (1910).
“Two gear tooth shapes are in common use, the cycloidal and the involute.” The idea is that the teeth should roll on each other rather than sliding.
“The obliquity of the twisted tooth if twisted in one direction causes an end thrust on the shaft, but if the herring-bone twist is used, the opposite obliquities neutralize each other. This form of tooth is much used in heavy rolling-mill practice, where great strength and resistance to shocks are necessary.”
Maximum tooth speeds recommended are 2400 feet per minute for helical cast iron gears, measured at the pitch circle.
The above was written about 1895.

Next up is to look at all the bearings.

My brother brought over the correct external lock ring pliers, Sears Craftsman 9-49273, he found them on sale in the clearance bin.

A modern equivalent snap ring would be McMaster Carr 98585A130.

I noticed the front seal housing is stamped SGK/34 and on the other side 4. What do you suppose that means, Standard Gear of Kilarney or Knightsbridge? Some of the gears are stamped JR.
The front bearing is Hoffman R140L but there is a shim tucked under it which I don’t yet understand. Not sure if somebody replaced the whole box or just parts.
Center bearing Hoffman RM 12-1/2L. Rear bearing Hoffman MS12. No discernible wear in them.

Needle bearings: 27 of them measure .118" and 31 of them measure .1175" so I’ll be replacing them all.
counter shaft gear ID is 1.218" both ends.
counter shaft at worn spots: front .978" and rear .9785"
So that calculates at from .0035" up to .005" slackness.
I think I’ve identified the trouble. Gear teeth would be engaging not quite at the intersection of pitch circles, would cause noise.

The counter shaft is either 38392 if it was not changed, or 48756 = C915 if it was.
Now to find out if the C1856 counter shaft sold by XKs and others will work.
Anybody know more about these?
Could the difference be the notch cuts on the rear end?

Here is an SH box. I wonder what is the reason for the second cut? Maybe used in some other model gearbox?

SGK perhaps?


The gearbox data plate is a separate one in brass, that is fitted just below the main plate. It is the same width/length as the main plate, so the fixing holes are the same spacing as the ones above. If you have evidence of two more holes, you are missing the plate. If there is no evidence of holes, maybe they didn’t fit them to all cars. Your chassis number indicates, I believe, a late '48 DHC in LHD and would have been made close to the same dates as my saloons, which both have these plates.

As an extra bit of trivia in this vein, the body number stamped on the main plate is repeated with another plate on the opposite side of the bulkhead. They were of steel, not brass, and many rusted away or corroded to an unreadable state, but easily repeatable if you have the right sized character punches. Except that the original had embossed characters, not impressed.

Hi Peter,

Yes, these gearbox plates do appear in some of the MkIVs but they didn’t exist in the SS Jaguars. The body numbers are present on the SS Jaguars and can be found on all the doors as well as the bulkhead.


Hi Rob,

The 2.5 litre later SS parts list drawing shows the same cog angles as yours and needle rollers.

Interestingly you have bigger gear teeth and fewer of them in your box, except for 1st gear where they are the same.


Peter L, you may have me confused with another, my chassis is 40051 a very early 1938 2.5 saloon, in fact the second car of that configuration for the 1938 model year according to JDHT.
There is definitely no gearbox tag nor holes for it, though there is a body tag.

Peter S, I have not found body numbers on my doors, though I have not yet taken off the rear interior panels to look there.
That drawing of the gears is the one I was suspicious about, as having done a lot of drafting and corrected many more by others, I know how easily a part can be drawn backwards. Check the drawing of the 2-1/2 Litre water manifold, upside down. But anyway he seems to have got the gears drawn right.
You’ve given me another idea, count the teeth and figure out the ratios.
More later.

Hi Rob,

My apologies about any confusion, there was an overlap between your message and Peter Scott’s. I was referring to the Mk IV plate only, in Peter’s photo of a few days ago and his enquiry to me. Peter’s photo does not go down low enough for me to see if there is evidence of whether a g’box plate was fitted, so I replied to explain where this data would be, but omitted to link this to your original question specific to the SS where you showed the number on the selector extension. Also, I am an experienced ignoramus when it comes to SS/IV differences, and with this number being the box number for the IV, I assumed it was also the case for the SS.

Just a bit more trivia. The two small flat rectangular bosses at the top corners of the casing are workshop reference points for manufacture. The rear one that is at the top left of the box for the bigger cars is longer and has the gearbox number stamped on it, repeating the one on the selector extension. The 1.5 box in the photos above show these to be too short to take the box number, and is similar to a colleague’s '48 1.5 IV.

I can’t see a 1.5 box anywhere in this thread!

Peter :worried:

What may be confusing you is that the top cover was changed during the MkIV to give a lighter gear change with longer lever travel. The SS 2.5 and 3.5 cars never had this top cover. The 1.5 MkIV continued to use the original top cover but with an angled lever instead of the straight one and at some point in Rob’s car history it has been fitted with an angled lever.


You can see from the above diagram that the Mark IV & V cleverly separates the leverage for fore and aft travel from that of travel across the gate. The red line represents the single fulcrum point of the SS gear lever and you can see that the centre line of the across gate leverage in the later cars gives a slightly wider gate but the fulcrum for fore and aft gives a very much longer lever travel.

This adventure is getting murkier and murkier. Is there now a suggestion that my gearbox could be from a 1.5 L?
The main cast iron case measures 9-7/8" overall length.
I checked again, no markings on any of the four corner manufacturing bosses. I understand their purpose, to locate the rough casting in the machining center before the first cut is made.

Here is the top cover and shift lever.

Here are the gear teeth counts and ratios.
gears.pdf (21.6 KB)