Listers,
Here's what I have. Good stuff for sure, but I think the question that
started this thread had more to do with rotors than calipers.
Rich Rock
Pottstown, PA
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Four piston brake caliper conversion for TR3/3A/4/4A/250 and 6
by R. John Lye and Lee Janssen
First, in our current litigacious society I feel that I must point out that
the neither of the authors are brake experts, nor are we automotive
engineers; as such, follow these instructions at your own risk. The
braking system on your car must function correctly to avoid serious injury;
if you do not feel competent to do these modifications correctly, please
either stick with the stock brake system or have a professional do the work
for you.
In the quest for better braking on our cars (autocross prepared TR-4, and
high performance, street driven TR-6), we began looking into four pot
caliper conversions. While there are several sources of commercial
conversions, especially in England, and there are race calipers like
Wilwood and AP that we could have used, we were looking for a simple, cost
effective solution. We discovered that the calipers from a 1979-83 Toyota
four wheel drive pickup (non-diesel) are almost a bolt-on conversion.
These calipers are readily available, and are relatively inexpensive; at
this time, they should cost about $39.95 at your local auto parts store
(plus about a $20 core charge). You will also have to buy a "hardware kit"
for a few more bucks to get the pins that hold the pads (one kit per car).
There are four things that need to be done to fit these calipers to the
Triumph uprights (late TR-4 through TR-6):
1. The Toyota mounting bolts are 12 mm, rather than 7/16", so our first
solution was to make up some reducing bushings to fit into the ears of the
caliper to bring it down to the 7/16 ID. These bushings have a 12 mm OD
and a 7/16" ID and were made from mild steel. However, there is a better
solution; there were three types of calipers fitted to the TR-6 range. The
first was the Girling 16P (which was fitted until 1969) followed by the
Girling 16PB (which was fitted until 1973). Both of these calipers had
"Imperial" threads. After 1973, TR-6's got the Girling M16P caliper, which
had metric fittings. So far, so good. The interesting thing is that there
are different mounting bolts specified for the different calipers: 113142
is specified for both the 16P and the 16PB (ie, the "Imperial" thread
types), while 158668 is specified for the M16P (these are the factory part
numbers). Here are measurements of the relevant diameters:
0.478", 0.479" Toyota caliper mounting bolt hole inner diameter (measured
2)
0.433" 71 TR6 caliper retaining bolt (113142) shank outer
diameter (measured 1)
0.472", 0.469" 74 TR6 caliper retaining bolt (158668) shank outer diameter
(measured 2)
The latter bolt has a metric (12 mm) shank but has 7/16" threads and it
fits the Toyota calipers perfectly. These bolts should work to mount the
Toyota calipers to the TR-2/3/4/4A/250 uprights as well as to the TR-6
uprights. I just wish I'd known this before I made up those bushings! One
drawback is that the hybrid metric/inch bolts are more than twice as
expensive as the standard bolts.
2. The bleeder bolt uses a 10 mm wrench instead of the standard 7/16"
wrench, so you'll need to buy one of those if you don't already have one.
3. If you are fitting these calipers to a TR-3/3A/4, you'll need to make
up a flex line that is SAE (or AN-3) at one end, and metric at the other
end. The caliper uses a 10 X 1.0 inverted flare fitting, which is a bit
hard to find as a fitting for braided lines. If you want to use a straight
end right on the line, Earl's part number 640803 will work (this should be
available from many of the race supply shops such as Summit Racing). I
wanted to have a 90 degree end, so I used a metric to -3 male to male
adaptor that I got from Porterfield (1-800-537-6842); their part number for
this fitting is 420103. To be sure that it sealed, I backed it up with a
dowty washer. If you are fitting these calipers to a TR-4A/250 or TR-6,
your job will be a bit easier. These cars use a short piece of hard line
that goes between the caliper and the flex line. For these cars, you can
simply buy a length of metric brake line from your local auto parts store.
Cut the female SAE end off your current hard line and cut one of the ends
off the new metric line (cut it to the proper length at this time). Put
the female SAE end on the piece of metric hard line and form a new flare
(be sure to make the proper double flare with a good quality flaring tool;
this is essential to get a good seal). If you are not sure how to make up
brake lines, be sure to get a professional to do this for you; it is
essential that the brake lines function properly.
4. Finally, the brake dust shield needs to be trimmed to clear the larger,
Toyota calipers. See figure 2 for a trimming diagram; the black areas
should be removed. Alternatively, the dust shield may be left off if you
prefer.
5. In addition to the above instructions, to fit the Toyota calipers to a
TR-3/3A or early TR-4, you will need to use the later brake caliper
mounting plates (Stanpart part numbers 133499 and 133500). These were used
on the late TR-4 through TR-6 cars, but they will bolt right up to the
earlier vertical links.
That's it. It is quite an easy conversion, and the pads are readily
available from all the usual sources (both standard-type and racing pads
are available) as this is apparently a common conversion for the Datsun
Z-cars.
We have performed some braking tests on this conversion. Braking test
summary is as follows:
The distances are based a 74 TR6 running at 3000 rpm in 4th gear with a
3.45 differential and 205-70 R15 tires. The tire weight distribution was
as follows:
L front 660
L rear 780
R front 620
R rear 720
Front wheel camber was set to 3/8 degree positive.
All tests were performed with a newly rebuilt master cylinder, using DOT 5
brake fluid.
EARLY (prior to metric calipers) Stock configuration with semi metallic
pads, composite shoes and 0.70" rear wheel cylinders: 262 feet with a
standard deviation of 19; 6 data points, tossed out both the minimum and
maximum measurements.
Toyota front calipers with semi metallic pads, composite shoes: 243 feet
with a standard deviation of 9; 8 data points, tossed out minimum and
maximum.
Toyota front calipers with semi metallic pads, composite shoes with a 7/8
inch brake cylinder:
215 feet with a standard deviation of 9; 9 data points, tossed out minimum
and maximum.
Observations:
Testing was much harder to do than I expected. Left front tire was about
1.5 pounds under inflated for all tests and would lock prior to any other
wheel for all tests; since this was constant for all the tests, this should
not affect the relative results. With the new rear cylinders, another wheel
was locking up shortly after the front left but I was uncertain which one
it was (I suspect that it was the right rear).
My test method was to start braking when a bar painted on the road
disappeared under the hood. Brakes were applied to start squealing the
front left tire but not to allow it to lock up.
Pedal travel increased as the size of the calipers/cylinders increased.
This was initially a little alarming but it was fine once you got used to
it.
Since the first car that this conversion was performed on had rear discs,
we weren't sure what we'd need to do as far as brake bias goes with the
drum brakes on the rear. So, a proportional valve was installed on the
rear brake lines of the TR-6 used for the brake tests above, but it was set
to have no effect (completely backed out). We had predicted that the 7/8
cylinder would overpower the rear brakes; however, this turned out not to
be the case. At this time, none of the cars that we are aware of that have
had this conversion performed on are using proportioning valves and none of
these are having any problems with rear wheel lock-up.
Sources for parts mentioned in the text:
Porterfield: 1-800-537-6842
Summit Racing:
The Roadster Factory:
Moss Motors:
Carbotech Engineering:
TSI:
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cheers,
John Lye
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