gfiandy Posted March 3, 2012 Share Posted March 3, 2012 Hi, I think I need to replace or refurbish the rear drums on my MK3 Spit. I can't adjust the hand brake to come on low enough without the brakes dragging for part of the rotation. I am guessing this means the drums are oval.Since these parts are not all that expensive I am happy to buy new but I hear lots of reports of new parts being poor. Does anyone know where I can get a good set or should I have the existing ones skimmed?Regards,Andy Link to comment Share on other sites More sharing options...
Nilfisken Posted March 3, 2012 Share Posted March 3, 2012 Hi Andy,just checking, you did remember to adjust the brake shoes first?Cheers,Oskar Link to comment Share on other sites More sharing options...
PJM Posted March 3, 2012 Share Posted March 3, 2012 Any local engineering firm will be able to skim them for you, maximum wear/sizes are in the manual. I doubt whether they will be oval, just worn although iof the shoes aren't centralised / adjusted correctly they will give issues. Link to comment Share on other sites More sharing options...
gfiandy Posted March 3, 2012 Author Share Posted March 3, 2012 Hi, I have adjusted the shoes as far as I can but even as they are now they drag lightly for about a third of a revolution. I can't think of any way the shoe adjustment could cause this, isn't it the case that if they drag for part of a revolution there has to be oval wear in the drum or off centre bearing which seems unlikely.Has anyone bought new drums recently and can comment on their quality?Regards,Andy Link to comment Share on other sites More sharing options...
Nilfisken Posted March 4, 2012 Share Posted March 4, 2012 Sorry, no new drums bought so no idea of quality of the new stuff :( Link to comment Share on other sites More sharing options...
Dannyb Posted March 4, 2012 Share Posted March 4, 2012 When you changed the shoes did you operate the foot peddle a number of times to centralize them. Link to comment Share on other sites More sharing options...
gfiandy Posted March 4, 2012 Author Share Posted March 4, 2012 Hi Danny, The new pads have been on for about 500 miles so should have had plenty of time to bed in. Oscar has very kindly lent me a spare rear drive shaft with brake assembly in place. Whilst it seemed like the drums were OK when we tried them on his newish shafts. With the drum tightened down on this spare shaft and the shaft in a vice they display the same symptom. So I think they must be slightly out of round. I think we have a dial gauge at work so I may be able to borrow that to check them. At least I now have a set up I can experiment with to try to track down the fault. Ill let you know what I find in the end.Regards,Andy Link to comment Share on other sites More sharing options...
lancepar Posted March 6, 2012 Share Posted March 6, 2012 Have you had the drums off and checked that the linnings are not becomming detached (if bonded) from the shoes, they try to straighten out and rub on the drum. :( 8) Link to comment Share on other sites More sharing options...
James1500 Posted March 6, 2012 Share Posted March 6, 2012 I had new drums last year, no problems so far, covered 6k since fitting Link to comment Share on other sites More sharing options...
gfiandy Posted March 17, 2012 Author Share Posted March 17, 2012 Ok I bought a new set of drums and tested them on the drive shaft clamped in a vice. They show the same problem.So I borrowed a dial gauge from work. I have now tested the drums the variation from round is 0.2mm. This seems quit high but there is no information in either the Haynes or workshop manuals on how much variation is acceptable. I checked the hub as well this shows only 0.04 mm variance so it is not the shaft being off centre. It could be that the drun is not mounted centrally however the drum is close to an interference fit on the hub so I can't really see how this could be the problem.The only thing I can think of is that the drum hole is not completely concentric with the outer. This seems like a major mistake on the part of the manufacturer and it's odd that the old and new drums show the same variation.Anyone got any other ideas.Regards,Andrew Link to comment Share on other sites More sharing options...
black6osprey Posted March 17, 2012 Share Posted March 17, 2012 I have exactly the same situation with my GT6 on both rears. Have you tried changing the pull off springs as this was my next plan. Link to comment Share on other sites More sharing options...
gfiandy Posted March 17, 2012 Author Share Posted March 17, 2012 I am currently working on a spare shaft off the car the pull off springs are keeping the pads hard against their seats so this should not be a problem.If this was causing the drag it should be for the whole way round.Not sure what to cheek next.Regards Andrew Link to comment Share on other sites More sharing options...
junkuser Posted March 18, 2012 Share Posted March 18, 2012 "I am guessing this means the drums are oval."(from original post)This assumption fits the bill Andrew.As you have the hub mounted in a vice, assuming the hub bearings are in good condition, you should be able to set up a piece of metal, or better a dial gauge, to touch the inside of the drum to get an idea of the amount of run out.Could do the same for the hub but not sure what this all would achieve apart from finding where the fault lies and picking the drums with the least run out to use.Could use an internal micrometer or vernier to check around for ovality.Not sure what the specs are for run out or ovality tolerance.Could set up axle between centres with drum mounted and machine the inside of the drum.(Ford America machined rear discs whilst mounted on there axle in at least some models. Had to replace axle and disc as a unit.)The selection of drums I have for the Spitfire are all oval to a degree but work OK if adjusted to just touch during rotation. Link to comment Share on other sites More sharing options...
gfiandy Posted March 18, 2012 Author Share Posted March 18, 2012 I have done this with a dial gauge, see earlier post. The drum shows 0.2 mm out and the hub only 0.04mm. However I can't find any information on what is acceptable. It may be that my expectation that the drum should not drag for part of the rotation is unreasonable.Regards,Andrew Link to comment Share on other sites More sharing options...
junkuser Posted March 18, 2012 Share Posted March 18, 2012 Sorry Andrew, I thought you were only measuring eccentricity rather than ovality.My mistake. Link to comment Share on other sites More sharing options...
black6osprey Posted March 18, 2012 Share Posted March 18, 2012 2674 wrote:I have done this with a dial gauge, see earlier post. The drum shows 0.2 mm out and the hub only 0.04mm. However I can't find any information on what is acceptable. It may be that my expectation that the drum should not drag for part of the rotation is unreasonable.Regards,AndrewAverage brake shoe to brake drum radial clearance should be 0.3mm Link to comment Share on other sites More sharing options...
gfiandy Posted March 18, 2012 Author Share Posted March 18, 2012 I thought it might be eccentricity but when I checked if the variation was 180 degrees out of each other I found it was actually about 120 degrees from min to max. I think this means it must be oval in the drum.But I am not a mech eng so if I am wrong I am happy to be corrected.I have now put the drums back on, on the basis they work and the only reason I started down this path was the hand brake could not be adjusted to pull on low enough not to stress the rubber handbrake cover. This has torn and I am now thinking the best way to resolve this might be to add something to cover the tear.Just got your information that 0.3mm is max deviation, my drums are within this so guess I will have to find another way to resolve handbrake. Thanks for the info, where did you find it ?Regards,Andrew Link to comment Share on other sites More sharing options...
black6osprey Posted March 19, 2012 Share Posted March 19, 2012 2674 wrote:Just got your information that 0.3mm is max deviation, my drums are within this so guess I will have to find another way to resolve handbrake. Thanks for the info, where did you find it ?Regards,AndrewA friend with a Spit got the info a while ago from KD Triumph in Coventry. I've since checked the internet and found reference to this for most drum brakes so I think its a pretty safe standard dimension. Cheers Rob Link to comment Share on other sites More sharing options...
gfiandy Posted April 8, 2012 Author Share Posted April 8, 2012 Just a quick update to show how I measured the drums. Using a spare drive shaft (thanks to Oscar for the loan) I clamped it in a workmate. Then using another drum as a stable bas I measured the deviation from round. I marked the high spots and low spots to see if they were 180 deg out from each other on the basis this would indicate off centre mounting.I did not find the high spots to be 180 deg out from the low spots.I measured two new Drums (the machining on which seems quite rough showing clear cutting ridges)AndTwo old drums which show no machining marks but are a bit rusty.The outcome was Max deviation on the old drums was 0.2mmMax deviation on the new drums was 0.3mmWith the new information that 0.3mm is the specification. I have accepted this is as good as its going to get and put the car back together with the old drum. The new drums appear to have been fairly roughly sand cast then machined the old ones were still sand cast by the look of them but possibly with finer sand they also show makers marks clearly inside showing the better quality of casting. The old castings have thinner walls and are lighter, presumably this will slightly reduce unsprung weight but provide a little less thermal mass. The older castings are definitely better machined. Since I have no plans to race the car I am happy with the lighter drums.So in answer to my original question are "New brake drums any good" I would have to say they are just about acceptable but not as good as NOS if you can find it. Link to comment Share on other sites More sharing options...
Nilfisken Posted April 8, 2012 Share Posted April 8, 2012 Glad to see it worked out and thanks for the report! Link to comment Share on other sites More sharing options...
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