Monday, May 14, 2012

Heat Sinking the Regulator/Rectifier

One of the weak points of the 1988-90 Honda VTR250 is the regulator/rectifier.  The rectifier converts the AC current, created by the motorcycle’s alternator, into DC current to charge the battery. Additionally, the regulator ensures that the battery is not over charged by “dumping” extraneous voltage to ground. In the process of regulating and rectifying, the unit can get very hot. Having it mounted just aft of the rear cylinder, within the VTR’s bodywork, is not helpful either. When a regulator/rectifier fails, due to overheating, it usually causes the CDI (Capacitor Discharge Ignition) unit to fail as well. Usually, there’s a distinct smell when these components fail (or “fry”) and there might even be some melted wiring.

As you can imagine, this is not an inexpensive issue to fix. It’s even more concerning on a VTR due to the fact that the regulator/rectifier is a rare object and commands a high price (whether new or used). So, in an effort to help the regulator/rectifier shed some of the heat it generates, attaching a heat sink would be beneficial. There are various ways in which this can be accomplished. The following details the steps I chose.

The image below illustrates the first dilemma. The image is oriented such that the valve cover, of the rear cylinder, is at the bottom of the picture. The red arrow indicates the location of the coils. The yellow arrow indicates the location of the regulator/rectifier. I estimated the distance between the two to be about ½” (0.5”). That’s not a lot of room in which to squeeze a heat sink (and have it be effective).

The second dilemma isn’t so obvious. If you look closely at the image above, you will notice that the side of the regulator/rectifier that faces the coils is the one where the circuitry is installed and sealed. This side of the unit will not be good at transferring heat. What’s the solution? “Flip the unit over and mount it the other way”, you say. Not so fast my friend. If you choose to do that, you’ll end up with the issue you see illustrated below. The yellow arrow indicates the gap, created by flipping the unit, between the mounting ears of the regulator/rectifier and the support bracket. Additionally, the mounting studs are now too short and there’s no room on which to thread the nut.

So, we need to figure out how to 1) secure the “flipped” regulator/rectifier to the support bracket, 2) secure a heat sink to the regulator/rectifier and 3) space the support bracket such that there’s room to accommodate the heat sink between the coils and the regulator/rectifier.

Reconfiguring the support bracket to accept the “flipped” regulator/rectifier

The first thing that needs to occur is to remove the existing studs that hold the regulator/rectifier to the support bracket. The studs are spot welded to the bracket. I used a hack saw to cut between the bracket and the base of the studs. The yellow arrows in the image below indicate the stud bases.

Once the studs have been removed, you can replace them with M6-1.0 x 25 button head screws (I initially used Allen head cap screws but that causes clearance issues with electrical connectors – so, use something with a low profile). Secure the screws to the support bracket with an M6-1.0 nut and M6 flat washer. The nut/washer combination acts as a spacer for the mounting ears on the regulator/rectifier (as indicated by the red arrow in the image below).

Securing the heat sink to the regulator/rectifier

I was able to find a heat sink out of an old personal computer. If you have an old PC lying around, it might have what you’re looking for. You could always check with a work station services tech at your place of business or ask the neighborhood ‘geek’ if he/she has an old computer you could pillage. Computers are very recyclable and most municipalities have recycling centers. I highly doubt they would mind if you pilfered a couple of machines for parts. The last resource is the internet. Lots of companies sell heat sinks. Depending on the material, they’re pretty cheap. You’re looking for one that’s about 1.75” (W) x 2.5” (L) x 1” (H). The key dimensions are the height and width. There’s not enough room for a heat sink with more than one inch of height. Additionally, the width must be less than two inches or it may interfere with the mounting screws. I had to cut mine down so it was 1.75” wide and less than one inch high.

Heat sinks are manufactured to be attached to computer chips so they’re already lapped (very smooth/flat) on the mounting surface. If you have to cut one down to size, be sure to keep the mounting surface free from gouges and burrs. I used some electrical contact cleaner and a piece of Scotch-Brite to clean the mounting surface of the regulator/rectifier. There’s more dirt there than you think. I chose to use Arctic Silver Alumina Thermal Adhesive (part # AATA-5G from, ~$8). It’s a two-part ceramic epoxy specifically formulated to bond heat sinks. Even though it comes in small quantities, you don’t need much. After scrubbing the regulator/rectifier, I cleaned the mounting surface with rubbing alcohol (per the Arctic Silver directions).

I wiped down the mounting surface of the heat sink with rubbing alcohol as well. I then mixed the epoxy per the manufacturer’s instructions.

The amount I used, total, had to be about the size of a penny (diameter and thickness). Arctic Silver recommends that the adhesive be the thickness of two sheets of paper (that’s not much) between the components. Since the heat sink is lapped, I applied the adhesive to that surface. The “pot” time (amount of time you have to apply the adhesive once it’s mixed) is only 3-4 minutes (very short), so work quickly. I put the heat sink and the regulator/rectifier together and, with a very slight twisting motion of the heat sink, made sure that the two parts had made good contact. I used one of those Vice-Grip Quik-Clamps to hold the parts together while the adhesive cured (initial cure is sixty minutes, total cure is about four hours). Now is a good time to go grab some lunch, do some laundry, check the scores, whatever.

After the initial cure, the adhesive has set enough so that you may put the regulator/rectifier back on the support bracket and install the bracket on the bike. You can reuse the Honda flare nuts to secure the regulator/rectifier to the support bracket.

To provide additional space, between the coils and the heat sink, I used longer hex head cap screws (M6 – 1.0 x 35) and a 1/4” x 3/8” x 1/2” steel spacer.

Here you can see the lower fastener with the spacer between the support bracket and the mounting tab on the frame. Note: the upper fastener captures a grounding lug from the wiring harness. Be sure that the lug is between the spacer and the mounting tab on the frame.

 Here’s everything in-place before the electrical connections were re-attached.

Now that the upper mount, of the support bracket, is spaced away from the frame mounting tab, make sure the wiring harness is not pinched between the support bracket and the frame. I simply pulled the harness out, away from the frame, so that it rests outward of the bracket. Here, everything has been re-attached.

The heat sink is indicated by the yellow arrow.

I estimate there is about .25” clearance between the heat sink and  the coil support bracket.  There appears to be about .5” clearance, after re-installing the side cover (bodywork), between it and the support bracket. Even if I push inward on the bodywork, it does not contact the support bracket. Problem solved (hopefully)!

Parts List:
2 - M6-1.0 x 25 button head cap screws
2 - M6-1.0 x 35 hex head cap screws
2 - M6-1.0 nuts 
4 - M6 flat washers
2 - 1/4" x 3/8" x 1/2" steel spacers
1 - Arctic Silver Alumina Ceramic Epoxy
1 - Heat Sink (approx. 1.75" x 2.5" x 1.0")


  1. My mention : it's a right improvement that weak part to exclude a dramatic ending after it crack.May be I repeat it at my Xelvis 1993.

  2. Hi, thanks for the comment. I've never heard of a Honda Xelvis but I did Google an image of it. I see it's a Honda VT250F which is very similar to the VTR250 Interceptor. There appear to be several regulator/rectifier alternatives for these bikes. However, if you can keep yours from failing, you'll be better off in the long run. Replacements seem to be expensive or require modifications (or both).