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Cymber
United Kingdom
966 Posts |
 Posted - 04/04/2008 : 20:01:43
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I need a replacement worm for my steering box but so far have only managed to locate T Type ones which are slightly slower. I therefore have two questions:-
Does anyone know of a source of the correct ones?
Can someone who has experience of the T Type worm in an MMM comment please?
Maurice Blakey.
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kimber
United Kingdom
1529 Posts |
Posted - 08/04/2008 : 23:40:19
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I too would welcome some good debate on this topic as I believe information is short on the subject.
I am slightly mystified about the different steering box ratios in my 2 P-types. One I know has the standard P-type Bishop Cam box. The other also has a normal Bishop Cam box with an apparently lower ratio worm gear. Neither box appears to have received any modification.
I know that early P-types used Marles Weller boxes and later ones Bishop Cam, but the question is did they change ratios in the Bishop Cam box during the production run? I vaguely recall seeing it written somewhere that this had been the case, I thought in W E Blower, but I have searched in vain.
From a driving point of view, the higher ratio box is much more precise, but heavier, especially at lower speeds although quite suitable for racing. The lower ratio box requires noticeably more movement of the steering wheel and on a tight turn this can mean shuffling your hands to find a bit of extra lock, which is generally not the case with the other box.
I am intrigued that both seem to use the same peg although I am not convinced that the profile of the groove is the same in both cases.
Herewith a photo of the two worm gears:
The coppery one is the 'standard' type - I have seen several like this. The other is the 'low ratio' one which was fitted to the car when I acquired it. I have been trying to make sense of the various numbers stamped on the worm gears. If enough people respond, maybe we can work it out between us.
http://farm4.static.flickr.com/3040/2399652168_153077a494.jpg?v=0
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Edited by - kimber on 08/04/2008 23:46:33 |
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ags
United Kingdom
275 Posts |
Posted - 09/04/2008 : 13:03:00
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Hi Andrew,
I believe that the photos of steering cams which you have shown are probably those of a PB and a PA, in that order on the photograph, ie the PB is the copperised one. One of the best features of the PB, in my opinion, is the "faster" ratio of its Bishop cam box. Blower does not emphasise this subject, but he does quote the box ratios as 9:1 for the Bishop PA (the Marles Weller box was even "slower" at 10 7/8:1) and 8:1 for the PB. The easy way to tell them apart, on the car, is to check the turns lock to lock. The PA box gives 1 1/2 turns and the PB is 1 1/4. On my car, which definitely has the right box, this figure is possibly closer to 1 1/3, but this does depend on lock stop settings etc. However it is definitely not as great as 1.5. Do not allow anyone to tell you that the only change to the PB was the bigger engine, there are a number of other things that are good improvements, but not many people think about them. This steering box and the better ratios in the gear box are just two off the top of my head.
The MG Part numbers for the two cams were MG443/103 for the PA cam and MG635/100. It is probable however that the numbers on your cams are Bishop numbers and I know of no reference source for those. Close study of part numbers in the PA and PB parts list is very enlightening! It shows that your assertion that the rocker shaft in both boxes is the same, for example. (If you think about the geometry of the box it is not too surprising, for as long as the upper arm is within a moderate range of lengths then it is independent of the form of the cam.)
My identification from the photograph relies on my memory, which is becoming more suspect, alas, and it could be that the "steel" cam is in fact a T type cam, which had a "slower" ratio still, apart from early TAs. The earliest TAs (I do not know what change point, nor any T part numbers) had an 8:1 box like the PB but I guess that this was rapidly found to require too much effort, and later TAs, all TBs and TCs used 11:1. Perhaps John James with a foot in both camps would care to comment.
However, I would be rather worried about using the "copperised" cam too much. My guess is that this finish is someone trying to take up wear in the cam, as the application of copper is a very simple electrolytic process. These coatings are very soft, though, are porous and possibly not very well adhered to the cam and therefore are likely to wear rapidly. The softness would be another advantage for the anonymous bodger as it means there would be no need for expensive precision grinding to get the cam to fit. A better, and longer lasting repair is to press the peg out of the rocker shaft because it is only a tight press fit and this can be done in a vice. It can then be turned through 90 degrees and pressed back. At the very least, depending on the degree of wear, this will move the area of greatest play to the extremes of lock and away from the central area which is the usual driving and MOT critical zone. Adjust the steering box shims carefully and you should be OK for many thousands of miles. If this is still not good enough then replace the whole peg with a new one. I know that these have been available, though not whether they are at the present.
I think that the only way to save the cam itself would be to first find a precision grinding shop prepared to tackle the cam. (And I am afraid that I do not know how to remove the cam from the shaft.) Then find an engineering plater and ask him to remove the copper facing, which will be no great problem. Following that you need hard chrome deposited to a thickness sufficient to take out the wear in the cam and allow a very slight excess for truing up by your grinder. If the wear is reasonable again this should be no problem. Bear in mind that that hard chrome can only be deposited in thicknesses of low numbers of thou.
You will note that I have said nothing of Yorvik's modified top to the steering box. I am not convinced by the principle of this, as it seems to me that it transfers the moderately high loads on the top of the rocker arm (from the wedging action of the sides of the cam) from the whole area of the top of the rocker arm to a localised patch. Indeed some while ago I convinced myself that I could feel the standard top of the box flexing as the steering was turned with normal loads on the road wheels. The lids of the boxes also score and wear proving that the loads are not light. Faced with these problems I tackled them by fabricating a new, considerably thicker (quarter inch from memory) lid from a piece of decent wearing steel. This was a simple job even for my limited manual skills. Draw filing made sure that the lower face of the lid was flat and thus both wear on the original lid and possible flexing were cured at once. Ten or twenty thousand miles later the repair still functions.
From your general question I have given a specific answer to one aspect.
More technical ramblings from,
Andrew Smith MMM571 |
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kimber
United Kingdom
1529 Posts |
Posted - 09/04/2008 : 14:37:47
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Andrew
Thanks for the detailed response. The 'copperised' cam is not intended for use. It is on loan and shown for comparative illustration purposes only. I have another which appears satisfactory.
I had reached similar conclusions to just about everything you say. It would be nice to 'break the code' of B/Cam hieroglyphics.
I have pressed one of these worn gears off the shaft in the past and fitted another. Next time I strip one down I will photograph the splined shaft which is worringly puny. I had to make up a rig to allow me to press the worm back on without bending the shaft. The worms do have a tendency to work loose on the spline leading to play. Loctite will work well to overcome this problem provided the wear is not too severe.
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Edited by - kimber on 09/04/2008 18:32:21 |
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Gerhard Maier
Germany
873 Posts |
Posted - 09/04/2008 : 20:16:13
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THE SPORTS CAR July 1939, page270,
gives the answer why the steering cams were copperised:
"--- After machining, the cams are
heat-treated and copper-plated, the
copperising ensuring that the finished
steering box will work smoothly
during the early life of the car. ---"
Gerhard |
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kimber
United Kingdom
1529 Posts |
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Cymber
United Kingdom
966 Posts |
Posted - 09/04/2008 : 20:47:45
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My PA's worm is as the left hand one in the photograph and has the same markings. This is a late PA first registered 26/06/35. I think my PA was 1 1/4 turns lock to lock, (I can't check it at the moment as it is in bits) Blower lists late TAs, TBs and TCs as 1 1/2.
When I enquired of Andy King recently about replacement worms he had only the lower ratio T Type ones but said that they should fit the earlier boxes as both the rocker arms and pegs are the same.
Mike Dowley has pegs on the shelf.
Whilst talking to Mike last week he told me that some time ago there were worms available some time ago with suspect heat treatment. He does not stock worms at the moment.
As to removing the worm from the shaft, I was referred to an article on the rebuild of T Type steering boxes. Go to www.tregister.org and click on the Totally T Type button. This shows one way of doing it.
Maurice Blakey. |
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John Reid
United Kingdom
704 Posts |
Posted - 09/04/2008 : 21:09:45
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Like Andrew S, I was concerned with the rather flimsy nature of the top cover which was also badly scored after many years' service. I made a replacement from 1/4" gauge plate, left unhardened but with a lapped finish. So far so good.
John R |
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kimber
United Kingdom
1529 Posts |
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Richard Hardy
United Kingdom
2159 Posts |
Posted - 15/04/2008 : 10:24:59
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quote:
Originally posted by Yorvik
I have been reading this topic with interest.
In response to a comment made by AGS regarding the modified Bishop Cam top produced by www.vintageMGparts.com the modified top itself does NOT transfer "high load to a localised point" as he possibly suggests. The spring pressure in the top has been carefully calculated on the shaft centre. The pressure between the worm and peg when turning is far greater than our production setting for the top. This modification is carefully set up so that just enough pressure is exerted to keep the peg in full contact position in the mid range on the worm section of the worm where the wear generally occurs in the Bishop cam box.
The arm lifts up from the worm when approaching full locks and as such the arm pushes the ball up into the top and the arm itself resuming its normal position riding on the cover plate as originally intended.
This modification top provides a smooth transfer from the worn section of the worm and must not be confused with previous high pressure peg and lock nut type spikes which when over-tightened by owners would often end up creating excessive wear to the worm.
Clearly there are other ways to remove play from the steering box and as highlighted in other postings on this topic, costs are considerable and finding the skills to undertake such work is not going to be easy. We have however designed an alternative simple and cost effective solution which addresses the problem where it is most required and one which will not create damage to the steering box.
Hope this helps.
Yorvik
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Edited by - Richard Hardy on 15/04/2008 10:30:16 |
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