Turning a 502 into a 503

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Turning a 502 into a 503

Postby Schoobytwo (Bruce) » Sat Jan 07, 2012 10:24 pm

Hi all,

From my experience, everyone's favorite Diablo is the 503 due to the electric start. The 503 design has several other desirable features. One is the Hirth 190R motor used in the 503 has a larger flywheel than the Hirth 54R used in the 500 & 502. This larger flywheel balances the motor better and makes for a smoother running machine. Also, the larger muffler on the 503 quiets down the Diablo a bit.

The problem with 503's are that they are getting harder and harder to find. But with finding some components and making a few changes, you can turn your 502 into a 503.

Lets start with the chassis.

The 503 has 3 motor mounts on the post rather than two used on the 502. The addition of the three new holes is easy as they will not interfere with the existing holes. Measure out the holes and drill them through the post bracket.

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Then take some 5/16 course nuts and grind off the hex corners. Make them about the same size as the holes you drilled in the post.
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Place the ground off nuts into the post and weld them in. Grind them smooth.
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Now your chassis is ready for the three motor mounts.
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Next, building the bottom motor mount support.
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Re: Turning a 502 into a 503

Postby Schoobytwo (Bruce) » Sun Jan 08, 2012 9:13 pm

Another addition needed to the post is the angled bracket with the rubber bumper installed. Because of the heavier engine within the 503, Bolens must have thought an extra support was needed should the motor mounts sag. I believe they were right as often even with 3 motor mounts, we find them sagging downward and the bottom motor mount resting on the bumper.

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This bracket was made to the same size as the original. The bumper is the same bumper used on the bogie wheels.

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Next - The Battery Box!
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Re: Turning a 502 into a 503

Postby Schoobytwo (Bruce) » Wed Jan 11, 2012 9:00 pm

Of course, a 503 needs a battery box. This simple box is spot welded around the outside with also a few plug welds in the inside.

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Not shown is the top hold down bracket which is also needed. I'll put this on when the battery gets installed. It's also worthwhile to note two other changes.

- As mentioned earlier, the 503 comes standard with a larger muffler. Theoretically, the Hirth 190R and the 54R motors are the same displacement at 300 cc with the same piston and same stroke. With that, I have seen cylinders with larger fuel ports within them which could explain the need for a larger muffler. However in most cases, the cylinders are the same. Knowing that, my opinion is either the smaller 502 muffler or a larger 503 muffler could be used. For this 503, I put the larger muffler on.

- My oberservation is that all 503's came standard with a chromed steel bumper and the 502's came with an aluminum bumper. For this 503, a chrome bumper was installed.

With this, the chassis is now complete.
Too many Diablos and parts to count.
Yamaha Apex (it outruns the Diablo for sure)
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Re: Turning a 502 into a 503

Postby Schoobytwo (Bruce) » Thu Jan 12, 2012 9:48 pm

I'm going to cheat a little on a few things. One being the handlebars. I'm using an original pair of 503 handlebars. However, if you were utilizing your 502 handlebars, you would need to cut them and have some tubes bent to look like the originals.

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The 503 uses a thumb throttle rather than a half moon. The half moon will work fine, but if you want this to look like a 503, then you will need to cut off the metal around the handle and install a thumb throttle.

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Weld a plate off the choke angle plate for mounting the Voltage Rectifier to.

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Lastly, drill holes in the center of the choke and key switch angle plates. The 502 will have two holes offset in each plate. The 503 only has one in each.

I'm also using a dash from a 503. But if you had a dash from a 502, all you need to do is epoxy the existing holes shut and then drill two new holes for your key switch and choke cable. The key switch will be different, but the original choke cable should work.

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Too many Diablos and parts to count.
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Re: Turning a 502 into a 503

Postby Schoobytwo (Bruce) » Sun Jan 15, 2012 8:59 am

When it comes to the motor, the original Hirth 54R in your 502 cannot be changed to an electric start. Therefore, a search for an alternative motor is needed. This is not as hard as one would think.

The original motor in the Diablo 503 is a Hirth 190R. This is a very rare motor and it's unusually small output shaft is what makes it so special. Not sure of the proper description, but I refer to this taper as a 25mm taper because it measures 25mm at the top of the taper. If you have ever measured your clutch bore, it is about 24mm at the larger part of the bore. So when the 24mm bore clutch is on the 25mm motor taper, the clutch hangs off the motor about 3/8" to 1/2". This offset aligns the Primary Clutch with the Secondary Clutch and give clearance so the clutch does not rub on the starter bracket bolt.

A far more common motor is the Hirth 191R. This is often found in Sno-Jets and a few other sleds of the late 60's and early 70's vintage. This motor is virtually identical to the 190R with only a few external differences. One being that the above mentioned taper is larger at 30mm. Second is the bottom motor mounting holes are larger. Inside, the motor has the same piston, same cylinder, same head and same stroke and is the same horsepower.

This particular 191R came with the cylinder head in the wrong orientation for our 503's. Below you can see the intake port (the lower port on the cylinder head) is on the right and it should be on the left. The ignition wires and the pulse port also come out the wrong side.

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This will require the head and cylinder to be removed. Then the piston will need to be turned 180 degrees. The arrow on the piston should point toward the exhaust port, or more importantly, the piston ring groove pins should be on the intake port side of the engine. This particular cylinder head has the large fuel passage design which will give me enough power to beat my buddy Tom in a race (shhhhhhh - don't tell him).

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While the head is off, swap the pulse port with the plug on the other side of the motor.

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Re: Turning a 502 into a 503

Postby Schoobytwo (Bruce) » Sun Jan 15, 2012 9:19 am

Once you have the piston and pulse port swapped around, reinstall the cylinder and cylinder head with new gaskets. The original motor came with standard nuts holding down the head. But for mounting within the 503, standoff nuts are needed.

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Torque the cylinder head down to 220 inch pounds using a lubricated standoff nut.

To change the ignition wire routing, the ignition plate will need to be removed. This particular 191R has the updated ignition design with the external ignition coil. All the 190R motors I have seen and some of the 191R motors will have the ignition coil under the flywheel. I presume the external coil design was utilized to improve the life of the components.

Mark the plate by scribing a line on the plate. Remove the plate a route the wires through the opposite hole in the casing. Then reinstall the plate.

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I like the terminal blocks used on the 190R, so here you will see I've duplicated that design.

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The top and bottom motor mounts from your 54R are different than those needed for this new motor.

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Too many Diablos and parts to count.
Yamaha Apex (it outruns the Diablo for sure)
Yamaha 600 SX
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Re: Turning a 502 into a 503

Postby Schoobytwo (Bruce) » Fri Jan 20, 2012 8:06 pm

An important part of a 503 is the Voltage Rectifier. Again, finding one of these original voltage rectifiers is almost impossible. The good news is, we have learned to improvise.

The voltage rectifier for this lighting circuit is a simple insulated 2 diode design with a fuse. Diodes are one way electrical components like a check valve would be in plumbing. They take the alternating current (AC) from the lighting coil and change it to direct current (DC). The fuse in the circuit is there in case a diode fails. Without the fuse, a blown diode would allow current from the battery to go back into the motor and it would burn out the lighting coil or worse.

Following are some very basic descriptions of what I do to modify a rectifier. To fully explain with examples would be a topic for a separate posting of it's own.

To start, I find Bosch Regulators/Rectifiers at swap meets and stuff. They all tend to look like this:
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We take the top off the unit and basically gut the inside leaving the single post in place and the terminals we will reuse.
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An existing insulated stud is replaced with a third spade connector.
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Then we add diodes and a fuse holder to the circuit and wire it up.
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Once updated to the circuit we need, reinstall the cover and you are ready to go.
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Too many Diablos and parts to count.
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Re: Turning a 502 into a 503

Postby Schoobytwo (Bruce) » Sat Jan 21, 2012 9:06 pm

Hopefully when you found your 191 motor, it also came with a starter. The original Bosch starter used on Diablos is almost as impossible to find as the original rectifiers and motors. The good news is, there are starters that work just as well but they just do not have the starter solenoid attached to the starter.

The original Bosch starter on the 190R is a 001 158 004. The starter that came with this 191R is a Bosch 001 160 001. Notice that it does not have the solenoid attached.

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No matter the starter you use, make sure the rotation is correct.

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For a starter solenoid, I attached a typical starter solenoid to the top of the bottom motor mount and routed the positive battery cable to the solenoid and then down to the starter.

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Too many Diablos and parts to count.
Yamaha Apex (it outruns the Diablo for sure)
Yamaha 600 SX
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Re: Turning a 502 into a 503

Postby Schoobytwo (Bruce) » Mon Jan 23, 2012 8:19 pm

The headlights on a 503 are much brighter than those on the 500 & 502. This is because the 503 has a battery and when the lights are on, there is always a full 12 volts going to the lights. In the 500 & 502, the only voltage generated is that from the internal lighting coil located within the engine. This lighting coil's output rises and lowers with engine RPM.

With this higher constant voltage, the lights on the 503 are completely different. So off you go on a quest to find headlights.

The first thing to understand is the lights are made by WIPAC which is a European company. Second, 503 headlights are a component cleverly hidden inside WIPAC 210 Reversing Lights. The lights you look for look like this:

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When you take the lights apart, inside you will find the bucket you need for the 503 headlights. The buckets look like this:

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Now that you have your headlights, you need to modify the hood to accept them.

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Then modify your nose cone by routing out the rectangles:

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Just like that, you have a 503 hood with all the goodies.

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Too many Diablos and parts to count.
Yamaha Apex (it outruns the Diablo for sure)
Yamaha 600 SX
User avatar
Schoobytwo (Bruce)
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Posts: 1710
Joined: Fri Feb 22, 2008 12:47 am
Location: Sobieski, Wisconsin

Re: Turning a 502 into a 503

Postby Schoobytwo (Bruce) » Thu Jan 26, 2012 10:01 pm

The 191 motor will need a heat shield installed. This shield will be different than your 502. Without the heat shield, the heat from the motor will cause vapor lock in the carb. With this addition, your original carb from your 502 will work just fine with this 191.

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Too many Diablos and parts to count.
Yamaha Apex (it outruns the Diablo for sure)
Yamaha 600 SX
User avatar
Schoobytwo (Bruce)
Site Admin
 
Posts: 1710
Joined: Fri Feb 22, 2008 12:47 am
Location: Sobieski, Wisconsin

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