Disc brake conversion on Frogeye

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Disc brake conversion on Frogeye

Postby Asolepius » Sat Jun 16, 2012 10:17 am

I have now restored all the bits from the 1500 Midget donor. Good grief, I have never seen anything so rusted. The pistons were seized solid and eventually came out with a lot of heat and brute force. Useless of course by then, so I have new ones now. The odd thing is that they had been fitted with the hollow side inwards, towards the disc, whereas the exploded diagram shows them the other way round. Which is correct, or doesn't it matter?

Sombody had assembled the calipers with bolts that were too short so the threads were horribly corroded. In fact all bolt holes were corroded, so a further GBP50 went on new taps to clean the threads.

I had all the parts blasted and some of them powder coated - the dust shields, the stub axles, and a spare pedal box which will look far better that the one I have. Needless to say in the process some inexpert masking ensured that the threads were plastic coated so again the taps came in handy. The worst part was getting the plastic off the kingpin bushes.

After blasting the discs, it was clear that one of them had been skimmed too much in the past. The other was OK, but it's madness to replace one disc so I have a pair of new ones. They were so cheap that it's not worth not doing. I toyed with drilled and grooved items but baulked at 6 times the price.

At the same time I have obtained the 1500 rear drum units and the backplates have also been powder coated. New cylinders were again cheap on eBay, but the circlips that came with them are useless - very thick and brittle so they are impossible to fit and one of them broke in the attempt. The old circlips were far better, but of course terribly rusted.

Sadly the suspension wishbones were pretty mangled, presumably by whoever pulled the units off the donor car, so I will use my existing ones. Hopefully I can get them off, blasted and powder coated (along with the springs and anti-roll bar) without too much delay. The Midget kit came with good fulcrum pins which may come in handy. The kingpin journals are quite corroded, but amazingly there is no play when assembled so I will quietly forget about that. I had this problem with the existing kingpins and had one of them welded up and machined back to correct dimension, then heat treated to restore strength. It would be nice if there were a Speedi-Sleeve of the right size but I could not find one (and they are very expensive).
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Re: Disc brake conversion on Frogeye

Postby hoggie » Sun Jun 17, 2012 9:48 am

Take a close look at your wishbones, especially near the fulcrum end. I found hairline cracks after I cleaned them up and started painting them. Had to replace them both. Nice thing was the replacements came with new fitted fulcrum pins.
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Re: Disc brake conversion on Frogeye

Postby Asolepius » Sun Jun 17, 2012 3:15 pm

Thanks Mike - in fact I had already replaced the wishbones, when I rebuilt the car. Worth another look of course.

I eventually worked out which way round the pistons go - the parts diagram is wrong. So were the instructions that came with the new seals. The calipers are all back together now, but I had to make a tool to press the outer seal retainers in. I can't see how that task is possible with the caliper assembled!

I got new bolts for the 2 parts of each caliper, but they are too short. So are the backplate securing screws. What is it with Moss? Not the first time fixings have been wrong.
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Re: Disc brake conversion on Frogeye

Postby Asolepius » Wed Jul 18, 2012 3:37 am

Latest progress:

Today I am picking up a piece of 7/8" 16 gauge aluminium tube, that will go into the master cylinder to sleeve it down to 3/4". My engineer friend says it will wear OK, and will be easier to de-burr after drilling the ports. It will be fitted using Loctite hydraulic sealant.

I have ordered new kingpins from Sussex Classic Car Parts. They are cheap at only GBP22 each including bushes. The old ones were not worn much but were heavily pitted with rust.

Yes, progress has been slow, but it has to be right. And there was an interruption to change the starter motor.
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Re: Disc brake conversion on Frogeye

Postby HAN5L778 » Sun Jul 22, 2012 1:55 am

I had a similar problem with caliper bolts from Moss or other suppliers. I have been using grade 8 bolts from a hardware store or bolt supplier, with Loctite. They are cheaper too! I also found circlips at a hardware store that work. Measure the diameter and width of the groove and you can probably find some. You will need the old, thin, waskers to back them up.
Ok, it helps that the hardware store I used was owned by a friend of mine that used to race a Sprite, and has several street Sprites so his inventory included things he, and friends, needed. The new owner, one of his employees, has kept most of those pieces in stock. But I have found other hardware stores in town that stock grade 8 bolts.
I also use banjo bolts for the calipers that come from a Harley Davidson motorcycle. They are the same thread and length as one from a Spridget, plus they are about $4.00 each instead of $7.00 each from Moss.
Richard
'69 Sprite-project with lots of trick parts.
'63 MGB waiting for restoration
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Re: Disc brake conversion on Frogeye

Postby Asolepius » Tue Aug 07, 2012 6:08 am

I now have almost everything ready for the conversion. The ally tube was slightly thicker walled than it should be so I have had to turn a tiny bit off the piston. That was delayed by the lathe motor exploding (well it was about 60 years old!), and fitting a replacement. I just had a thought. I have a spare master cylinder and it would be convenient to have that ready to fit instead of rebuilding the one in the car. If I sleeve down both bores, will I have to change the clutch slave? I imagine I will have lower clutch pedal pressure but it will take a bit longer to bite fully. Now the lathe is working I can of course make a another 3/4" piston.
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Re: Disc brake conversion on Frogeye

Postby Asolepius » Sun Aug 19, 2012 9:53 am

I am nearly there. I decided to sleeve the existing master cylinder and that went OK, although drilling the ports and de-burring them was tricky. I now have the discs on both sides, and found that the toe-in is rather extreme! I used the existing steering arms and they are identical to the 1500 Midget ones except for the hole for the track rod end. I can't see what has changed the geometry so much. I think I need to add at least half an inch each side. Not sure if there is enough thread on the track rod end for that. Any ideas folks?

I'll post pics when finished, as I am rather proud of all the powder coated suspension parts!
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Re: Disc brake conversion on Frogeye

Postby Charlie Tolman » Tue Aug 21, 2012 12:54 am

Could the toe-in be due to a different length of the tie rod ends ?
There are different part numbers for the 948 and 1500 applications.

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Re: Disc brake conversion on Frogeye

Postby Asolepius » Wed Aug 22, 2012 5:46 pm

No Charlie, I am an idiot. Top trunnions were on back to front! :oops: Corrected in 10 minutes by lowering the lower trunnion onto an axle stand to compress spring and undoing all the top trunnion nuts. Geometry is now correct!

After what seemed like hours of bleeding fluid, I went for a test drive today. The brakes pull up square and strongly, with lower pedal pressure than I am used to. But the pedal is quite spongey. Will this improve as the pads bed into the new discs? The pads are not new but have no visible wear. Although I haven't changed the rear brakes yet (no time before a major outing on Saturday), I did bleed them as well as the fronts. As usual the clutch took seconds to bleed off the air, and works perfectly. Not sure I can get any more air out of the brakes.

Front suspension feels very nice now, with poly bushes all round. Ride not noticeably harder.
1959 Sprite restored at last - after 15 years!
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Re: Disc brake conversion on Frogeye

Postby Charlie Tolman » Wed Aug 22, 2012 11:28 pm

I initially thought about the upper trunnions, but if they are installed incorrectly, the first observation usually is the very large positive camber of the front wheels/tires.

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Re: Disc brake conversion on Frogeye

Postby Asolepius » Thu Aug 23, 2012 3:40 am

Yes of course, the huge +ive camber was what drew my eye to the trunnions! I shall do a few hard stops from higher speed later today and see what happens, but if the pedal doesn't harden up the only option is to do more bleeding. This is very unusual - over 40 years I have never had trouble with air in brakes, but then I only had drums before.
1959 Sprite restored at last - after 15 years!
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Re: Disc brake conversion on Frogeye

Postby Asolepius » Mon Oct 08, 2012 9:13 am

All done now, including the 1500 Midget rear brakes. These had the effect of stiffening up the pedal a lot, and enabling the pedal to be adjusted to the right position in relation to the others. The only unexpected thing - well 2 things really - was that the handbrake pull rods were wrong. I had to weld up the clevises at the backplate end and re-drill them smaller, and lengthen the rods by welding in sections. It was all a lot of hard work, and not particularly cheap, but it's the last major mechanical job I will ever do so good to get it right.

One disappointment was that I could not use my beloved Minifin alloy drums on the back, as the backplates are very different. So I had to restore the iron drums and use those. The Minifins have served me very well on the front for 7 years but the pedal pressure was getting a bit too tiring. If anybody wants them they are up for sale.
1959 Sprite restored at last - after 15 years!
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