BW35 Tech Miscellany

[enigmas]

Having recently rebuilt the BW35 in my modified MK3 P5 coupe, I thought I'd start posting some miscellaneous tech info in one place from my archive. Hopefully the information will be of use to any enthusiast either rebuilding or sorting this particular transmission. Although sited in the Modified section of the forum, the transmission is applicable to both the standard P5 MK3 and P5B as well as numerous other makes of cars, ranging from 4, 6 and 8 cylinder engines (up to 289ci XR Ford Falcon) that I'm aware of.

Some of the tech here may include beefing the transmission using components from the later model BW40 and BW51 found in OZ. Most of these parts can be fitted to the standard OEM Rover BW35 cases without modification.

None of the information is meant to turn into a debate (not interested)...it's just what I do, not necessarily what you should do! So if the info is relevant to your needs or helps resolve a problem, that's great.

There'll be no particular order as to the info presented so it's not an A - Z of rebuilding the transmission.

[enigmas]

BW35 Governor

There are several versions of Governor in BW35 transmissions. Most look similar to the one presented which is from my MK3 transmission. It's overall appearance will be similar to the P5B version apart from the governor weight which may vary in size.
The pictures illustrate all of the components assembled and disassembled.
















NB. Ever wonder why your BW35 sticks in 1st gear when cold and first moving off and perhaps after warming changes gear normally. It only takes a small (miniscule) amount of grit or stickiness to jamb the slide and or weight. The fix is straight forward, remove the tail cone, undo the 2 screws retaining the slide valve and moveable weight. Simply clean the removable assembly thoroughly checking that the components freely move when re-assembled. It can all be done in situ (yes it's a PITA) but the governor does not need to be removed from its shaft.

[nz]

Just as an aside, and a bit of a laugh, I knew a guy who could 'fix' a sticking governor on a BW35 without any tools. He'd drive the car at about 30 mph, and throw it in park. The resultant violent ratcheting would shake the governor free. (If it didn't actually lock the wheels!) Not recommended. Paul. NZ.

[harvey]

Throw it into PARK on the move it will lock the wheels if you're lucky, otherwise it will snap the pawl, but the one thing it won't do is ratchet.

[nz]

I have no reason to fabricate stories on this forum. I've seen this ratcheting trick done several times. I'm done here.

[harvey]

OK, bye.

[enigmas]

Mar 4, 2018 19:06:34 GMT nz said:

Just as an aside, and a bit of a laugh, I knew a guy who could 'fix' a sticking governor on a BW35 without any tools. He'd drive the car at about 30 mph, and throw it in park. The resultant violent ratcheting would shake the governor free. (If it didn't actually lock the wheels!) Not recommended. Paul. NZ.

Hi Paul, when I fitted my spare replacement trans after the broken case incident with my coupe...I had one issue...yes the governor would stick initially. I rebuilt the trans over 10 years ago and stored it under a bench. Before fitting it I did check some components and also modified the case slightly to take a BW51 3 bolt rear servo...but that's another story. What I didn't bother checking was the governer...a 10 min task on the bench. Storage and governor valves don't seem to harmonise!

I thought it would clear itself with use...but to no avail. I found quite a few comments in relation to 'rattling' the weight and slide free by placing the car in Park once doing over 50 mph. The pawl is supposed to not engage the locking teeth but ride over the top of them. I don't believe I have the balls to attempt it or risk damage to a fully rebuilt transmission. I'm willing to attempt it on someone elses donor vehicle though!

Physically dismantling the governor slide and weight cured the problem instantly. It was a quick task (once the tailcone was removed) to clean 'residue' from the surfaces of the moveable parts and restore its normal function.

I acknowledge that you are an experienced transmission rebuilder Paul but I doubt I'd have the nerve to free a governor using the 'rattle' free method.

[harvey]

BW65/66 parking pawls ratchet, presumably altered because of the number of broken pawls on the BW35, lots of which I've sold to remedy the problem, but don't worry I still have quite a few left if you want to try it.......

[enigmas]

Harvey the parking pawls on the BW51s (behind OZ Ford big 6, grunty efi engines) built into the 80's are virtually the same as the 35s including the cases. The differences include larger gear shafts, improved one way sprag clutches (more sprags) and the 3 bolt rear servo AFAIK.

Why not include/post some interesting tech specifics/trivia of yours here...it is after all, for the benefit of all who contribute to the Forum and the healthy maintenance of our Rovers.

[harvey]

Mar 5, 2018 0:44:47 GMT enigmas said:

Why not include/post some interesting tech specifics/trivia of yours here...it is after all, for the benefit of all who contribute to the Forum and the healthy maintenance of our Rovers.

I have a head full of things but I'm not able to retreive them just like that. I see other posts and they may remind me of something, and if there's a comment worth making I'll make it.

[enigmas]

BW35 VALVE BODY 'Primary Regulator Valve' Spring

This info relates primarily to the P5 Mk3 BW35, but don't just get hung up there as it can also apply to other vehicles using this transmission. There are some simple upgrades to improve it's performance and durability as many components are interchangeable between the various 'newer and older' model versions.

I recall my standard MK3 and the quality of the gear changes it provided when I first got the car. Once moving it would virtually jump into top gear and stay there, regardless of incline or throttle position. Gear changes were generally soft, mushy and any performance from the transmission was virtually non existent. A range of things can be done to improve both the transmission and car's overall performance in easy stages. Most can be done with the transmission in situ but some like improving the front clutches torque capacity can only be done with the transmission on the bench.

Below are a variety of Primary Regulator Valve springs as fitted to various cars utilizing the BW35 trans. Note the differences. All are a direct fit replacement. Note the spring used for MK3 PRV. Longer springs (increased tension) improve overall system pressure and thus clamping of the clutch packs and servos. Less slip results in less heat and subsequent wear. Shifts will also be firmer.

[enigmas]

Reverse Boost Device

Here's a device fitted to some BW35 valve bodies often in larger engine capacity options. These devices were often fitted to Leyland P76 V8s and were standard fitment to OZ Chrysler Centura (4 litre/245ci engine)

This mechanism fitted to the valve body assembly increases line pressure in reverse to 620-965kPa or 90-140 psi) at idle.

[enigmas]

2 - 3 Shift Valve Plug

Ever wonder why some BW35s will part throttle shift or drop back into 2nd gear at light/moderate throttle...not kickdown.

It's partly due to a redesign of this plug, providing a larger surface area. Note the difference in diameter between the 2 plugs. The smaller plug is the early version as fitted to the MK3 and P5B (AFAIK).




Sited in the upper removable section of the valve body.

[enigmas]

Throttle Cams

There are also several versions of the throttle cam. Note the differing profiles. The cam to the right raises system pressure more having a direct affect on the 2-3 shift plug that enables the part throttle shift to occur.

[enigmas]

BW35 & BW51 Rear Servos

Here are 2 rear servos. On the left a BW35/40 servo and on the right a BW51 servo. The piston diameters are the same for both. The main difference is the 3rd bolt hole.

The 51 version will fit straight into any BW35 casing and the rear two bolt holes are also a direct fit. The remaining bolt hole can be adapted to a 35 case to further secure the servo. Why do this...because behind a torquey engine the case can crack due to the servo flexing on its 2 rear mounts.




NB. When rebuilding your BW35 transmission check for hair line cracks where the original servo bolts to the case. This area is quite fragile, so ensure the servo adjustment is checked on occasion. Also ensure that reverse is not engaged with a high idle speed as the shock of clamping may precipitate this issue.

[enigmas]

Broken Case

[enigmas]

I'm going to continue adding tech trivia here on an ad hoc basis from former and recent escapades with these transmissions Richard. BW35s stood up very well behind original factory Leyland P76s and grunty 245ci Chrysler hemi's. I've never dismantled either of these transmisions or inspected the cases to see if the castings were stronger. All the upgrades I've carried out were on old (1966) English cases primarily MK3s using OZ Ford internals up into the 80s. As for my broken case it did last for at least 10 years or more in regular daily use.

Although your transmission is in lovingly original condition it would probably benefit from a careful strip and rebuild, simply to maintain its integrity. Most of the bushes from the pump back are usually fine but it's the ones that are forward in the pump that wear. The input shaft can be quite wobbly within the pump. Perhaps in some ways an unwanted rebuild but cheap insurance to maintain the trans in good order.

[enigmas]

Front Clutch Diaphram Spring

This component is found in the Front Clutch of a BW35 trans. It sits in front of the large apply hydraulic piston. There are several versions of this diaphram spring. This one has a C designation (second image). If your unlucky during dismantling you may find the diaphram spring broken in several places.

[harvey]

Mar 16, 2018 10:08:16 GMT enigmas said:

If your unlucky during dismantling you may find the diaphram spring broken in several places.

At a guess I'd say seven out of every ten (if not more) of those have been broken in boxes I've dismantled.

[enigmas]

BW40 Tech

I've been looking for a few spare transmission components for a while and recently found a complete BW40 transmission for sale at an 'old' car wreckers (Rusty Ford Wreckers) in Melbourne's western suburbs.

Here's a couple of pix of the box with the trans pan removed.







It was very grotty on the outside but the internals appear unmolested and in good shape. There's negligible end/side play on the front pump shaft and the pump nose bush seems good too. It came with torque convertor (looks to be a reco unit) and bellhousing that I don't require.

[enigmas]

BW40 Tech_Front Servo Specs

Here's some info you won't find if you query the 'expert trans' specialists or locate in any specific W.S. Manuals relating to this later iteration of the BW35.

I've posted it here as a resource for any enthusiasts who want to improve the automatic shift quality of their BW35. Specifically, the BW40 allows a part light throttle downshift from 3rd - 2nd and from 2nd - 1st. It is a much more throttle responsive transmission than the BW35 found in any factory standard Rover P5 or P6.

The added responsiveness is to due to upgrades found in both the valve body and front servo of the transmission. Both these components are a virtual direct swap into any BW35 case that doesn't have the rear pump included.

AFAIK the P5 MK3 is the only P5 to incorporate the rear pump in a BW35 case. Now having said that, my P5 MK3 Coupe hybrid runs the BW40 gear train and modified valve body including the rear pump in a Rover Factory original BW35 case. So it's all capable of being upgraded if the desire is there.

Here are the front servo details.

The servo part number stamped on the servo is shown as:
0546-900
149-9

The front servo bore size is 2 3/8" or 60mm
The realease port has a restricted orifice of approx 3mm staked into it.
The release port incorporates a small steel free floating ball.
There is a small bleed hole of approx 1mm or 1/16" on the back of the port.
The servo uses the lightest release spring internally.

NB.
You may also require the 'slightly' deeper transmission pan of the BW40 to clear the larger diameter front servo...or a pan from any BW35 that utlilizes the large front servo will suffice. Failing that, some carefully directed heat and reshaping with a ball peen hammer where the servo contacts the pan will solve the clearance issue.

* See pix below to reference these details.







Note the small bleed hole.




Note the small steel ball in the centre of the release port. This will move from one side of the port to the other depending upon whether the servo is being activated to apply or release.


Note the insert staked inside the release port. It reduces both the port orifice size and contains the small steel ball within the port.

[enigmas]

FYI

Checking the BW35, BW40 or BW51 Trans dipstick for Correct ATF level.

The transmission pictured below is a BW40 from an Aussie XF Ford Falcon.
Apart from some Valve body variations, Friction material upgrades and the removal of the rear pump they are virtually the same as the first BW35s manufactured.


The image below illustrates the trans dipstick held beside it's locating hole.
Note the Red and Green markers on the dipstick.

* Green is where the fluid should reside when the trans is warmed and running.
When checking the fluid level ensure it is in the Green Zone.
The top of this Green zone is approximately level with the trans pan gasket.

* The Bottom Red zone indicates too low a level of ATF.

* The Top Red zone indicates an over fill situation.




PS. (No unsolicited pendantry thankyou, I know the differences. This information is purely placed to assist those concerned with where the fluid level should reside in a properly warmed and running BW35 transmission )

[enigmas]

More BW35/40/51 Trivia

The car in the image below is an XF Ford Falcon Panel Van. Note the build date on the compliance plate. Pix below. (Sept 1992)
* The transmission shown on the bench of the previous post (directly above this one) is a BW40 from a similar make and model Ford.

BW35 transmissions were upgraded to BW40 in OZ, then in their final iteration, to BW51 up into the late 90s AFAIK.
The XF Falcon van shown runs a BW40. The gear selector has 5 positions. PRND21
BW40 internals can be fitted to any of the early UK BW cases, for example MK3 P5s and P5Bs.
They have improved friction materials in both the clutchs and bands with wider drums.
The BW51 geartrain is not a direct fit in earlier cases AFAIK. It is a slightly heavier build geartrain.
The BW51 sprag (more individual sprags in the one way clutch) is a direct fit into any BW35 trans.
The BW51 rear 3 bolt servo can be retro fitted to early BW35 cases with some direct modification to the case and the 3rd mount of the servo.
Gear train components are not interchangeable between OZ built transmissions and UK built transmissions. The front pump (3 sizes) and rear pumps are though.

ATF
ATF for these 'OZ built' transmissions is the current iteration of Dexron. The latest version now a full synthetic.
Dexron ATF is not suitable for earlier UK built (frictions & bands) transmissions. Dexron has a 'friction modifier' inherent in the formulation.
ATF designed for the UK built transmissions (non friction modified) can be used in OZ built transmissions but will result in a very firm race type shift.







Note the build date Sept 1992.


Engine sizes were either 3.3 litre or 4.1 litre.
Later models were Efi.

* This particular vehicle belongs to a friend and has covered 148,000 kms (Approx 91,900 miles)