SC12 Overhaul ...

I know this topic has been thrashed, but I am going to join in with my 2c worth.
 
I am returning my AW11 SC to factory spec, and the SC12 is a problem - lets face it, who's isn't .. The SC12 is a very basic design, it's only moving parts are the lobes, they are connected by a couple of 1:1 gear set but basically it's driven at a ratio of the harmonic balancer and the pulley. The lobes are Teflon coated to about .3 - .5 mm (this is really thick).. There are 4 bearings and these are the problem. There are 3 different sizes, 17x46x21 in the gear end, and 17x15x 14 & 18 in the pulley end. There are a couple of odd design decisions made by Toyota - the smallest bearing also has the highest lateral pressure as it has the pulley tension on it. All bearings are held in place by a groove in the bearing outer and case which is injected with epoxy. This is more than likely used to stop the aluminium case expanding and releasing the bearing when hot. It took me near 12 ton to break the epoxy and release the bearings, and I think I may have popped out a poo when I was nearing the 12t mark waiting for the massive bang....
 
Anyway, most of you would already know all this - and you would also know that the bearings are very unique and even though I have Koyo, SKF, and a couple of other part numbers, are unobtainable. However - my plan is this - I have located very high precision bearings with massive specs that are available with the 17mm shaft size, they are larger in diameter. I will be machining the casing to fit these and as they are slightly narrower than oem it allows for a rebate in which I will put a retaining clip.
 
The lobes tolerance is important as they must be a beez dick from each other and the case - over time with dust, vibration, swelling etc they wear - but only on the outer edge and by no more than about .1 mm. Leaving this would not result in a massive boost drop, maybe a psi or so. I intend to re-Teflon the outer edge as a 'topup'. The main thing is balancing, I want to balance the lobes laterally to increase the precision, and mainly to reduce any vibration.
 
With the new bearings and their high spec (good for 50K RPM continuous), and with the balanced lobes. I see no reason why the SC12 cannot hit 20K RPM reliably. It will depend on the amount of heat generated, but this is should be lower with good internals.
 
That's my plan - I will keep this thread going with updates - then if it's all successful, I may be able to offer a rebuild service, or at least all the info to DIY...
 
Specs:
 
SC12 produces 1.2L of air per revolution
1:1 @ 7000 engine RPM = 8.4L/Min = approx. 8 PSI (AGZE Config)  (SC12 RPM = 7000)
1:1.3 (avg pulley upgrade) @ 7000 engine RPM = 11L/Min = approx. 10 PSI (SC12 RPM = 9100)
1:2 (potential) @ 7000 engine RPM = 16.8L/Min = approx. 16 PSI (SC12 RPM = 14000)
 
The std computer would not handle this boost - the motor would run lean..
 
 
 
 
 
 
 

I will be watching with interest too as I have a couple of SC12's stashed away which I would like to rebuild as spares. Have you already found someone who is able to recoat the lobes?

If that should happen to be the case, put me down for 2 SC's straight away.

spike10000
I will be watching with interest too as I have a couple of SC12's stashed away which I would like to rebuild as spares. Have you already found someone who is able to recoat the lobes?

dennis the menace
A quick thought off the top of my head . . . is it going to work out cheaper per unit to do a run of lobe re-coating? There may be a few more SC owners out there who could be in for a group effort on this project.

Jon Bond Performance does re-coatings for the Eaton range of superchargers, might be worth contacting them to see if they could also do the SC1x's
 

Anyone willing to part with an SC12 ? - I need another for parts ?

Really looking forward to hearing more on this proposed project. There is someone on MR2oc.com doing some SC12 rebuild work as well, but they gave up on trying to do the case bearings as being too expensive. They do however, do some nice work on the pulley's and the pulley bearings. You might want to look at what they are doing on those parts to see if you can incorporate that into your work as well. If the case bearings can be done though I might look for a second hand SC12, pull it apart, and ship the necessary parts to have your work done, if the shipping doesn't kill me, lol. Would be worth it, to me, to have a near perfect SC12 in the '89 SC I have otherwise restored.

I've found bearings that may work.. they are 2mm larger than the oem (OD), ID is 17 so the shaft size will not need to alter .. I'm having issues finding a machine shop who has accurate enough machinery to CNC the housing .. the tolerance is very small 0.01mm max!
 

Epoxy is clever, but I'm planning on getting a rebate cut in the housing so a retaining surclip can be used - similar to the idler bearings (which spin faster too).. The bearings are considerably narrower so there is room for this.
Ironically the rear bearings are in most cases ok - they have very little load and run completely sealed with the high quality gear lubrication which also acts as a coolant- I think they may be an everlasting item. The main load bearing is also the smallest - it runs dry and has little protection from dirt. This one and the one next to it (this has petrolium based lub - or it did when it left the factory) die, so are the ones I'm looking to replace.
 
As for teflon coating - if you takes out the rotors and look at the lobes you will see that they are scrapped and worn. 3M have an awsome teflon product that is almost DIY - One big problem with teflon is you are trying to get something that doesn't stick to anything, to stick to the rotor .. The 3M product contains an adhesive agent to you dont need to bead blast the primary surface .. It may be possible to lightly sand the lobe ends, apply the coating and seal it in a domestic oven (slowly to temp and slowly cooled, must be fan forced to create an even temp) - I also have a selection of computer controlled IR ovens used for the manufacture of SMT PCBS - with these I can create a heat/time profile. Teflon bakes at about 250 degrees so not too hot!
 
I don't think we will be able to completely rebuilt the SC12 back to factory spec, but if we can do it enough to get another 10 years (maybe with a loss of half a psi) and for less than $100 then I think we all would be happy with that!...
 
The other possibility is to source an equivalent spec SC .. I see Rotrex (They make awesome SC units) have just released a little one that is clutch operated - identical to our SC12 - If the volumetric/rpm characteristics are similar to the SC12 then it would be simple to fit it as they are the size of a small alternator.