m6 bentley manual gearbox issue

Gary Andrews · Joined: 23 May 2016 Posts: 34

Hi

got an issue with my mk 6 manual gearbox

pulled away from lights went to change into second some thing felt as if it broke and no gears after a few seconds of fiddling with gearlever I got into second I think but could be first now lever in fourth position and stuck there so always in second or first wont move anywhere

any help with diagnosing fault would be appreciated

I suspect Ill have to remove box or can you access from top gear selector bits as I recon its something to do with that

Gary

Christopher Carnley · Posted: Mon Aug 28, 2017 2:17 pm Post subject:

Gary,

Chassis No?

1) Broken/shattered 3rd speed thrust washer, ruins the main shaft and 3rd speed synchro cone, and may damage the lay shaft.
2) Broken early main shaft splines, RG 193, b*****s everything

Gary Andrews · Joined: 23 May 2016 Posts: 34

Chris

chassis B332 BA

Christopher Carnley · Posted: Mon Aug 28, 2017 6:55 pm Post subject:

Gary,

Very early, 47/48 well before the DZ series strengthening modifications. You may have big problems.

Chris.

Gary Andrews · Joined: 23 May 2016 Posts: 34

I had read about upgrades to shafts etc sure mines never had then shes only done 26 thousand miles from new

If I take side cover off will I be able to see the problem ?

Christopher Carnley · Posted: Tue Aug 29, 2017 8:17 am Post subject:

Very unlikely.
I had an R Type gearbox from a guy in London 3 weeks ago, shattered washer, mutilated main shaft. A recent acquisition, 1953 car, he asked when the problem may have occurred, I told him that the washer may have first cracked across in 1954/5.

The use of SAE 30 grade engine oil was a big mistake and led to the sticking on of the synchro cones and consequent breaking of the foolishly thinned main shaft splines, and synchro lugs.

Chris.

Gary Andrews · Joined: 23 May 2016 Posts: 34

Chris

many thanks for your comments

Ive found the issue in the selector mechanism and have resolved

a run up the road shows all gears working fine again

Christopher Carnley · Posted: Tue Aug 29, 2017 3:04 pm Post subject:

Excellent news.

Richardson Masten · Joined: 01 Jan 1970 Posts: 49 Location: California, USA

Had the same problem on my R-Type. Tried the simplest solution first, and lubricated the selector mechanism. I've driven more than 14,000 miles since with no additional problems.

John Murch · Posted: Tue Dec 12, 2017 5:42 am Post subject:

So is this problem caused by the use of SAE 30 oil in the early days, and would all owners of these cars benefit from some flushing out of the gearbox?

Christopher Carnley · Posted: Tue Dec 12, 2017 9:14 am Post subject:

John,

With such an early gearbox there is no telling what modifications or repairs have been done.
The early milled out mainshaft splines was a serious mistake, not resolved until 1949, the early thin 3rd speed washer was never resolved on produstion until 1954, broken washer fragments wreck the lay shaft gears, but the biggest error was the 30 grade oil, again not "officially"changed until the Service Bulletin of 1952.

Owners still slavishly refer to the hand book as "correct" SAE 30 grade oil.

There is the persistent view that these gearboxes were weak, not so, but there were weaknesses, however the majority received scant attention and I know several spcial builders who are building cars around totally unknown, never opened gearboxes.
An expensive mistake when they wreck them and it has to come out again.
The 30 grade oill should not have been in any gearbox for the past 65 years.

Simon Philip Kelleway · Posted: Sun Jan 26, 2020 10:14 am Post subject: Bentley Mark VI gearbox oil

May I just ask then please...if we shouldn't use SAE 30 oil in the Mark VI gearbox, what should be using? Our car is from 1948....

Thanks, Simon

Christopher Carnley · Posted: Sun Jan 26, 2020 12:56 pm Post subject:

From Service Bulletin No 147 in 1952, the engine oil was discontinued and SAE EP 80 then superseded the 30 grade due to wear.
It was a mistake to specify SAE 30 in the first instance and that was to try to reduce transmission drag on the net 100 BHP output, as they did in the 1930,s.