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6 cyl Bristol cars Type 400 to 406 - restoration, repair, maintenance etc

B400 - overdrive oil

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Old 23-05-19, 08:52 PM
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Default B400 - overdrive oil

I wanted to check the level of the overdrive oil because it leaks.

The access to the level screw seems to be difficult - any hints?

But what kind of oil needs an overdrive?
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Old 24-05-19, 04:07 AM
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Bela ,
as you no doubt know , no Bristol 400's were fitted with overdrive originally and in my opinion not necessary in a well tuned and maintained original car.
What type of overdrive unit have you fitted .
Is the overdrive oil and gearbox oil common to both ?

Geoff
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Old 27-05-19, 07:57 AM
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Hello Geoff

I'll take a picture as soon as possible. There are two separate cases and the overdrive has a screw with a stamp "level" near by.
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Old 17-04-22, 11:13 PM
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Now I know what kind of overdrive it is.

Laycock J-Type - #25 B400 reconditioned by ORS Repair service Sheffield.

A German Workshop for repairing overdrives told me that it is special because of the filler plug. The recommended oil is KRON 90EP. Also ATF 3 may be a solution.
The big plug at the bottom is not for draining. To change the oil the base plate must be take off.
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Old 18-04-22, 12:03 AM
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Bela ,
I have had no experience with a J type overdrive but most overdrives used in other cars have a common gearbox and overdrive oil of about 30 grade .

I would not use EP90 as it has additives which could make the clutches in the overdrive slip .

I suggest you try a Gearbox oil of 30 grade without friction additives .
"Penrite" has these possibly in different grades .

If you can not locate any gear oil , try a cheap Mineral 10W/30 engine oil.

But my suggestion is not a recommendation as I have not used a J type.

Surely others on this Forum are using the J type overdrives and can advise!!!

Geoff
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Old 18-04-22, 06:23 AM
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Hello Goeff

Kroon Oil EP90 is suited for vintage vehicles and without modern additives. The Laycock J-Type had been introduced in 1972. I guess that was the reason for ATF3 recommendation - but the gearbox was not filled with ATF Fluid. With the new oil it works smoother.
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Old 18-04-22, 11:55 AM
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I have used Penrite Gearbox oil 30 for many years in my 403 with a 405 gearbox and overdrive.

I have to disagree very strongly that a well tuned and maintained 2 litre doesn't need an overdrive. The manufacturer clearly saw a benefit when updating from 403 to 405. I haven't driven an 85c with overdrive but the 100A has enough power to make it worthwhile. There are several 400s that are now sporting the very elegant conversion that Derek Hughes pioneered. It would be good to hear some feedback from anyone using one of these.

It may well be that we drive on different road types, but for UK diving with a fair bit of motorway mileage I feel the upgrade to overdrive was a huge improvement. It is vital to change the differential ratio to 405 spec to get the benefit.

The main benefits are lower revs, improved MPG and quieter cruising at motorway speeds but it also makes the car a bit quicker in the lower gears due to the change in differential ratio. I also recommend the use of the magnetic switch that was used on the 405 to drop overdrive as soon as a change down from fourth is made. The vacuum hold switch used on the MGB is arguably better still but these are hard to find now.
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Old 18-04-22, 12:26 PM
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It wasn't me who decided. I bought the car with this overdrive. If it would be needless I would not use it. But exceeding 55 mph while driving on the autobahn or flat roads temps me to switch into overdrive to enjoy a smoother ride.
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Old 24-04-22, 06:03 AM
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Default J Type Overdrive

Indeed Derek Hughes devised a system of fitting the J Type electric overdrive to the Bristol motor and gearbox and supplied all parts required. The system relied on removing the existing gearbox extension which contained the original sliding mechanism to the tail shaft, replacing the extension with a transitioning casting and the J Type overdrive unit and replacing the tail shaft with one with a sliding yoke.
The lubricating oil recommended by Derek Hughes for the J Type Overdrive unit was Castrol Transmax F automatic transmission fluid.

I have one of these units fitted to my Bristol 400 and a Bristol 403. They both work flawlessly with the recommended lubrication to the correct level.

Last edited by pnaleahy; 24-04-22 at 06:07 AM. Reason: typo
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Old 24-04-22, 06:42 AM
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Default J Type Overdrive fitted to Bristol 400

I now attach drawing of the oil level and filling arrangements on the Derek Hughes supplied J Type Overdrive. As can be seen from the sketch, the oil level in the overdrive is less than half full.
Attached Images
File Type: jpg LaycockOverdriveOilLevel.jpg (114.2 KB, 14 views)
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Old 24-04-22, 07:15 AM
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Thank you for this useful information. My box seems different. But now the oil variety is no longer a miracle. In D Ravenol ATF Fluid Type F will be a match.
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Old 24-04-22, 08:59 AM
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Quote:
Originally Posted by pnaleahy View Post

I have one of these units fitted to my Bristol 400 and a Bristol 403. They both work flawlessly with the recommended lubrication to the correct level.
What solution was used for the final drive? I changed from 403 differential to 405 differential but the latter were quite hard to find. Did someone come up with new crownwheel and pinion of the correct ratio?
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Old 24-04-22, 11:33 PM
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Default 403 J Type Overdrive

The J type suited the 403 rear axle and at 100 kph reduced the engine revs by about 500 i.e from 2500 to 2000 approximately. It is only possible to engage it in top gear due to an electrical cut-out switch which was incorporated on the gearbox. On the other hand, the 405 overdrive had different gearing and required the 4.2 ratio diff to suit that gearing.
As far as I can see, the Laycock de Normanville J Type overdrive system was devised to reduce engine revs/increase speed. On the 403 a steep incline required the overdrive to be taken out of gear to increase engine revs rather than strain the engine in top.
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