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preselect gearbox

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davidian
Posts: 99
Joined: Sun Feb 17, 2019 11:32 am
Location: la vinuela spain

preselect gearbox

Post by davidian »

I have read every thing I can about these boxes and understand how they work but there seems to be a few contradictions which I do not understand concerning adjustment so perhaps more knowledgeable members can fill me in. The forward gear anulusses sorry anuli all seem to be of the same diameter and the pull rods the same length so why does the pedal rise to different heights when a gear is engaged. The automatic adjustment shortens the pull rods but adjusting for slip lengthens them ( and screw in the stop screw). When the gearbox was new did the adjuster table strike the stop screw or did wear have to occur before the auto adjust came into play .The only problem I am having with my gear box is top gear which will engage when changing down from overdrive but is a bit hit and miss changing up from second so possibly a selection problem.The instructions for adjusting the selector gear is to put the gear change lever at its two extremities which are normally reverse and top, on the S S its reverse and overdrive ( which occupies third gear place) would it be better to move the gearbox lever to top when adjusting the rods
Regards David Ian

qantasqf1
Posts: 299
Joined: Mon Mar 07, 2016 10:13 pm

Re: preselect gearbox

Post by qantasqf1 »

David, the reason for different pedal travel is pull rod adjustment which varies according to the gear engaged. As you suggested, the brake operating mechanism is identical for all gears (except for a slight difference in the reverse and - I think - top gear hooks), but each gear demands different degrees of toggle action which accounts for the variations in pedal travel. The lower the gear the greater the toggle action because of the higher torque being applied to the annuli by virtue of the lower ratio; conversely the higher the gear the torque applied to the annuli progressively decreases. As far as linkage adjustment in concerned, you should adjust it with the gearbox lever set in reverse and then overdrive. i.e., at maximum travel.

davidian
Posts: 99
Joined: Sun Feb 17, 2019 11:32 am
Location: la vinuela spain

Re: preselect gearbox

Post by davidian »

Hi.Yesterday I removed the top cover from the gearbox to make a visual inspection and it was apparent it had been " tinkered " with in the past judging by the burrs on the slots of the pull rod nuts, also the springs on a couple of the gears looked to have been fitted wrongly. looking into the box from the passenger side the two end loops are attached to the right hand peg but following the wire where it passes round the left hand peg it then terminates as the lower loop on some but the upper loop on others so I suspect the auto adjust is not working, The ends of the pull rods are well below the face of the nuts with about half inch or more thread showing, except top whose nut is screwed down so the pull rod end is a fraction above the face of the nut , My question is that during the normal lifetime of the box how many turns would the auto adjust apply to the pull rod nuts and could my top gear problem be down to the toggle going " over centre " because of the nut being screwed right down ?
Regards David Ian.

davidian
Posts: 99
Joined: Sun Feb 17, 2019 11:32 am
Location: la vinuela spain

Re: preselect gearbox

Post by davidian »

Hi again. I just went out again to inspect the fitting of the adjuster springs and they all start and terminate on the right hand peg, I think to gain the wrapping action required the lower loop should be put on the left hand peg , encircle the nut and terminate on the right hand peg, in this position the pull rod nut does turn when the pedal is used but did not with the spring in its as found position , could some one clarify if I am correct.
Regards David Ian

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migray
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Posts: 353
Joined: Fri Feb 26, 2016 10:31 pm
Location: Bakewell

Re: preselect gearbox

Post by migray »

I have found that setting up the gearbox using the gauge shown in the workshop manual works very well. Different distances are specified for each gear, you adjust the nut by hand until the distance between the bus bar and the top of the case is correct. I used the end of a metal barbeque skewer as a depth gauge with distances marked on a sticky label attached to it. The distance is measured as a depth between third and top gear positions. A simple plate can be screwed in place above these positions to help locate things properly, each gear is selected in turn, the distance adjusted until correct for that gear. The gear changes are now very smooth. I do wonder how useful the automatic adjustment is, it is probably easier to check distances once a year and adjust by hand.

qantasqf1
Posts: 299
Joined: Mon Mar 07, 2016 10:13 pm

Re: preselect gearbox

Post by qantasqf1 »

Viola! As you can see, the looped end fits over the front pin of all gears.
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davidian
Posts: 99
Joined: Sun Feb 17, 2019 11:32 am
Location: la vinuela spain

Re: preselect gearbox

Post by davidian »

Thank you Steve and Mike for your interest, I have now re fitted all the adjuster springs in their correct position and pumped up the pedal , this resulted in the nuts being turned a quarter turn or more in all gears except top ( whose nut cannot move because it appears to have been screwed down the maximum amount). As the stop screws dont seem to have been tinkered with ( they are pretty much in the same positions as the photo in the Conquest workshop manual ) I will leave them alone and concentrate on top gear which is the main problem .All this unwelcome " tinkering" was done while the car lived in California ,as neither I or the previous owner have meddled with the gearbox. Do you think I should undo the top gear pull rod nut a few turns in order to see what the self adjustment does or is there a danger of the operating pin falling out of its cup on the pull rod if I do this ,I could then measure the bus bar height of this gear.
Regards David

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migray
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Posts: 353
Joined: Fri Feb 26, 2016 10:31 pm
Location: Bakewell

Re: preselect gearbox

Post by migray »

David
As your main problem seems to be selecting third when moving from second I think it is most likely to be slight inconsistencies in the rods and joints from the gear selector. I would make a small adjustment in the length of a rod at one of the ball joints and see if it improves or worsens the situation. One turn is quite enough. The ball joints do wear so there can be play there. The bushes on the bell cranks also wear causing some sideways movement. Standard oilite bushes can be used to refurbish these. Further adjustment is possible by slightly bending the selector arm on the gearbox so as to change its effective radius but I would concentrate on the rod length at the moment.

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