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sam christie
United Kingdom
3058 Posts |
 Posted - 13/01/2008 : 01:07:35
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A couple of questions for the experienced engineers on a topic which never seems to be fully answered-
I have just read that it is sometimes possible to have a camshaft reprofiled at much less cost than the price of a new one.
I was given an unused camshaft which had been damaged on the surface by a spilage of battery acid (but rust might cause similar problems). Is it possible, practical and economical to have a camshaft resurfaced ?
And a suplementary question - Can a worn lobe on a camshaft ever be satisfactorily repaired in an effective and economical way ? What about hard chroming or metal spraying ?
Sam |
Edited by - sam christie on 13/01/2008 13:58:54 |
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Ron Grant
United Kingdom
160 Posts |
Posted - 13/01/2008 : 16:00:09
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Sam,
Hard chrome is not a sutable material to use to build up any reasonable thickness. It is used as a very hard surface but with a negligable thickness only. Equally, my understanding of metal spraying is that although a thicker deposit can be sprayed, it would not be very sucessful for the high loadings of a camshaft and certainly not suitable where there would most likely be a "join" between the original camshaft material and the new metal sprayed area.
The "old" way of resurfacing worn camshafts was by depositing Stellite by the oxyacetalene welding process and then grinding back. Somewhat difficult to deposit without getting gas pores, if I remember correctly.
I think you would be lucky to find someone to cam grind the profile back on again these days.
A process that was used on the valve gear, probably only the rocker fingers, of MG racing cars in the day was Fescollising (probably spelt that incorrectly) This was an electroless nickel process carried out by the company, Fescol, who have I think long since gone. They also carried out hard chrome plating on rocker fingers as the normal hardness was not sufficient to prevent serious wear when the triple valve springs were used on the Q types.
Ron Grant |
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Bruce Sutherland
United Kingdom
1547 Posts |
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David Allison
United Kingdom
665 Posts |
Posted - 16/01/2008 : 11:58:57
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Re-profiled cams are a very good stop gap for repairing a camshaft which is not too badly damaged.
However some of the cam grinders do leave a little to be desired with regard to the accuracy of the profiles and the relation of the individual cam lobes to each other.
This causes problems with getting the cam timing spot on(although on most road cars this is not a major problem).
A new cam will always last longer and give a better overall performance - however there are some very good companies which provide a good re-profiling service.
I have a new camshaft in my NA which has been in place for 8 years, it replaced a cam fitted by my Dad (another new cam) which had lasted around 15-20 years and around 30,000 miles.
In the 60's Dad used what he could - the re-profiled ones used to last anything from 1 race to 1 year depending on the quality before the performance would dip.
You pays your money and takes your choice - I would always prefer to use a new cam - however at the moment having just surfaced out of a major lower half engine rebuild - I can not afford either!
If you car is a pretty standard road car - give the re-profiled cam a run - if you are racing - go new.
Regards David |
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talbot
United Kingdom
718 Posts |
Posted - 16/01/2008 : 12:25:32
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David,
If, as part of regrinding, metal is removed from the tip of the lobes will this not require new, longer valves to set the correct clearances ?
Cheers
Jan T |
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David Allison
United Kingdom
665 Posts |
Posted - 16/01/2008 : 14:20:15
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Jan
Valves are normally supplied over length and need to be shortened anyway.
The set up of the overall valve timing changes completely when the cam is re-ground and re-timing the cam shaft has to be done very carefully - this is of course the same with a new cam as well.
However on the re-ground cam the basic base circle is now a smaller diameter and the overall pitch of the cam is also changed.
That said the best re-profiled cams take this into account and as long as the basic timing is correct then all should be ok.
Problems occur when the grinder makes a mistake with the individual pairs of cams along the length of the shaft.
If he makes a mistake here - then the cam timing can be out by a number of degrees cylinder by cylinder along the whole cam.
Dad had 1 cam in the 60's which was out by 10 degrees by the time you got to number 6 cylinder (he sent that one back) - this started him on making new cams.
The modern practice is to submerge arc weld a high carbon material onto the cam, then machine the basic profile before tuftriding the camshaft and then finish grinding.
A number of the modern racing camshafts for Fords and BMC cars are done in this way.
Hope this answers your query.
Regards David |
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talbot
United Kingdom
718 Posts |
Posted - 16/01/2008 : 15:58:44
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David,
Thanks for the info. It's really down to costs - A new set of valves will probably be about ú80 so this must be added to the cost of a cam regrind. If the journals are worn these will also have to be kissed up and undersized white metal bearings cast and fitted. The bottom line is, as you suggest, it's probably best to buy a new shaft. As an aside I have a M Type cam that has been built up with stellite and reground. I am fitting a new one to my engine so if anyone wants the repaired one for a budget rebuild they can have it free gratis.
Jan T |
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