I have three of the 09-25 transmission drums that have pitted band surfaces. They are close to factory tolerance they just got wet years ago when Hurricane Agnes swamped my grandfather's garage in 1972. How much can I safely turn off them in order to clean up the rusted surfaces?
Unless they are very badly pitted I would not worry about it. Post a picture of your drums so that we can see what condition they're in.
I agree. If they're smooth enough to be used as is, I wouldn't take off any material. Let's see them.
If I had though drums I would polish them but not put them in a lathe and take off any of the surface.
The drums are pretty rough but they have very little wear. That's why I want to salvage them. Heres's a picture of them.
I would chuck them up and use fine sandpaper to clean them up a bit. If they are too rough they'll eat bands in a hurry.
I used emery cloth on mine. The pits, in my view are oil reservoir's Just leave em.
James, drums should never be turned down. It is nigh on impossible to get an acceptable finish that way. They should be ground. Opinions will vary on how much they can be ground down.
Allan from down under.
You could kiss them with a file and then polish with emery cloth. As others have indicated there is no excess thickness on the transmission drums. If you turn them on the lathe they will look prettier but be more fragile .
The transmission rebuilders I know all turn down the drums. I've done it on all my transmissions. Problem is not so much wear or pitting but the unevenness of the surface. Eg, dished, angled, not concentric with the bushing.
I would not turn the drums down at all. They crack easily if any material is removed. I would bead blast the surface rust and then use them. Kevlar bands will eat into them in no time. No need to remove any material.
I'm not thinking about turning much off.. Just about .010" off the surface. Enough to remove the rust and scale and reduce the size of the pits. But I do like the idea of glass beading. I could then give it a good polish and go with it. I run Scandinavia bands and I'm worried about shredding the band material.
Definitely glass bead them, then use a sanding belt. Remember, if you take .010 off, you are reducing the diameter by .020!
I have never built a transmission with out turning the drums, and that is a lot of them.
Model T drums are just like wheel break drums, if the drum is not true, the band will not preform to its fullest.
Never take advice from some one that has never done it, or has done it wrong. It still sounds good when giving advice even if all you have is an opinion, with out any experience.
After truing drums and saying there in not enough drum left is incorrect, at least.
True the surface, leave any grooves after that, and polish out with emery, that's it.
Just polish them
But the question was as it has allways been,how much can you safely remove?? Maybe the question should be how thick is the mininum thickness allowed? Bud in Wheeler,Mi.
I agree with Herm's approach. I doubt the minimal removal as he does (for truing) raises the risk significantly for cracking. I also doubt he ever sees issues with band chatter with drums that have been trued to zero and made to run concentric with the shaft. I see no harm in his method and only upside reward.
Could they be metal sprayed or hard chromed?
I respect your experience, but would never turn a drum down. The one in your photo looks like it was really nice before you machined it. That little bit of pitting would not affect anything. I can't fathom why you would remove material from a perfectly good drum like that.
Honestly I don't think I would use that grooved drum in James' photo at all. Good drums are still too easy to find. In the last month I tore apart two T transmissions, and both have nearly perfect drums. Also, with the drum being so rusty, what is the condition of the gears?
Is there a consensus on how thick they actually are? I don't recall any one ever mentioning that. Kohnke seems to shave them on a regular basis without any apparent problems but most here just advocate polishing a bit. What were they when new? Kind of rough or smooth? Rivet worn grooves I understand but not the pitting you usually see. I don't get why that would form in normal useage so I'm assuming the came with pitting from the factory.
It's not going to measurably weaken a drum to turn a few thousands off to get them round. In fact, doing so will add longevity to a drum by making the friction surface uniform and round. Where you will get into trouble is trying to remove grooves caused by rivets. There was a HUGE tolerance, as machining goes, for the diameter of the drums and most are not concentric to the bushing bore.
Drums may be easy to find but good drums are not cheap. Too many think they have gold mine in their pile of scrap metal. If they were so numerous and available all over, you could buy them for $5-$10 to keep them out of the scrap yard. And a new one surely wouldn't be worth $650.
The pitting means the drum sat outside and got wet and rusty. I don't start a transmission build with anything that was ever wet and rusty.
Original drums are as cast, not too shiny and not pitted either.
I stand by my recommendation. Low drums are especially thin. The transmission bushings have about 0.005" clearance so they all float a little bit. The transmission band is anything but precision and the drum is stationery when the peddle is engaged. The bushing determines the gear concentricity and if the drum is a few thousands off the band won't care. If James just cleans them up ad polishes them as best he can they will be OK. Use will continue to polish the surface.
They're not turning them gain concentricity. They are just getting a better surface for the band lining to run on so the rust pits don't act like a cheese grater on them.
How thick they are when made is kind of hard to say. They are made from castings and depending on how accurately they were positioned when first manufactured, the crude, "as cast", inner surface may have a fair amount of eccentricity to the machined outer diameter. So, the thickness will/can vary from one side of the drum to the other. I'm sure Ford must have specified a minimum allowable wall thickness. A trip to the Benson Ford may reveal what that was. No, the pits weren't there from day one. As Royce states, they are from rust. Unless it's very superficial, they need to be cut away or polished over. Deeply pitted drums should not be used. With recutting, it shouldn't necessary to get the drum surface 100% clean. Take as little as possible to get most of the heavy stuff off. I wouldn't take more than .005/.010 per side. But, that's just my thoughts on it.
I have turned drums for a long time never had a problem with cracks on a later date. Many times helps considerably with balancing some are way out of concentricity, don't bury the cutting tool into the metal enough to clean up. The bands love it particularly the reverse, have had more problems with reverse needing lot of pedal pressure than the others , problem solved
Guys... This has been great and there is a lot of good information here - regardless if there are differing opinions. It's all good.
The transmission is a spare that my grandfather had in his stash of parts to keep the 1925 Coupe running that he bought in 1948. I still have that car and just decided to clean up the spare transmission parts. They got rusted when they were cought in a flood back in the 70's. The coupe got submerged in 4 feed of muddy water too, and you can see the water line in the gray cloth interior.
In any event, even after all the opinions and information presented above, I still plan to turn the drums, but I will be very careful to balance how much is removed with the need. I will probably end up turning off about .010 from the surface and live with whatever pitting is left. It's not costing me anything since I'm doing it at work in the evening so we'll see how it goes.