The Bultaco Sherpa T trials bike



In 1974 I bought a Model 125 Bultaco 325cc Sherpa T trials bike. I rode it with the New Mexico Trials Association and worked my way up to the Intermediate class in our local trials. That doesn't mean I was good, just better than I had been as a Novice. I sold the bike when I graduated from university and moved away from home.

Here are a few ancient snapshots I found of me riding in an NMTA event circa 1974/1975. I have very few photos from my early riding days.:

As documented in a different page I resumed riding trials (vintage now) on a Kawasaki KT250. I can't claim to be able to outride the KT, but I did remember with fondness my old Bultaco, with the very heavy flywheel mass and effortless power.

I normally avoid looking at eBay or Craig's List as I have trouble resisting temptation. I experienced that problem again 11 August 2008 when I brought home a 1976 Bultaco Model 159 325cc Sherpa T trials bike.

The bike seemed pretty stock, other than bars, some "Assault" rear dampers (never heard of those before), a Mikuni carb and Sun rims. I don't know if those are accessory plastic fenders or not. One short fin is broken on the right side of the cylinder head but I think that is mainly a cosmetic issue.

The under engine frame tubes are bowed upwards a bit and it needs sprockets and a chain. The tank has been sealed before and is seeping just a bit . The owner was telling me how it had been sealed and how it didn't leak a drop, and then I pointed out the rubber cushion at the back of the tank that was damp with fuel.

The guy I bought it from just trail rode it around on private land, he's never seen anyone else ride a trials bike. He had it set for about a 1200 RPM idle speed.

There is no owner's or service manual or any special tools with it so I'll have to find out what the best choice for a manual will be.

I've still got the Bultaco fanny pack that came with my Mo. 125 back in 1974. I'll look forward to wearing it when riding the T.

Later in August:

I've dissasembled the chassis and found a few issues. That Mikuni carb is 34mm, which was probably a reasonable choice if the bike were going to be motocrossed. I ordered a 26mm Mikuni from Bob Ginder at B&J Racing. The airbox appears to be a Sammy Miller item as it didn't match the parts book illustrations. I got a new air filter (with integral hose) along with various minor bits and manuals from Lynn Mobley at bultacoparts.com.

I've pushed the bottom frame rails back down off the engine cases. I've got new sprockets and chain on hand as well as a set of Michelin tires. The rear wheel needed 7 spokes so I ordered a complete new spoke set for it from Matt Hilgenberg at Speed & Sport

These hubs do not have a conventional cast iron friction surface in them. Instead, Bultaco hard-chromed the aluminum hub. It appears this either holds up fine -- or it doesn't. The front wheel is fine, the rear hub has some missing chrome. I'm talking to Langcourt about having the drum nikasil coated but the wheel looks good enough to use until the season is over. The hub is very light. The bare hub (no bearings or anything but the casting) is a little under 3 lbf. A sheet steel Maico rear hub is about 5 lbf and a BSA B50 conical rear hub is 8 lbf!

Bultaco also made the inner swing arm pivot bushings from hard-chromed aluminum and they are missing a lot of chrome. I've made replacements for them from some 25mm Thomson linear shafting which has a 60-62 Rockwell case-hardened and ground outer surface, but the center is still soft enough to easily drill and machine. I use a trapped needle thrust bearing race for taking the end thrust. There are steel bushings available from the aftermarket that are all one piece. I wanted the hardened and ground surface and since I don't have precision grinding equipment the 'built up" method is what I used. With some lubrication it should last a very long time. They do end up being 3X the weight of the stock aluminum parts, but in this case I'm willing to go for the longer-lived part. The bronze bushings in the swing arm seem in very good shape though I may add a couple of small grooves to them to help spread the grease around.

The fork stanchions have about .002-.003" total indicated runout which, other than making sure that they are both oriented with the same side to the front or back, I'm not going to worry about.

08 September 2008: I have had trouble getting the wheels aligned, and with the rear rim pulled as far over towards the sprocket side as possible the wheels are still a good 6-7 mm out of alignment. Time to measure things again, and here's part of the problem:

I clamped the swing arm to the base of the frame fixture as in and pried/clamped on the free end. I overshot the mark one time, and when I had reduced the offset to .025" instead of .25" I decided that was probably about as close as I was going to get with the equipment I had on hand. I was still seeing a lack of alignment between the front axle and the lower triple clamp when sighting down them. I removed the triple clamps and put the frame on the fixture, made the swing arm pivot shaft perpendicular to the sides of the fixture base and then used a dial test indicator to see how parallel it was to the base. There is a total change of .006" which seem likely to be well within production tolerances, so I don't think the frame is bent. Next stop, investigating the triple clamps.

Yes, the lower triple clamp is bent.

I was just down in the garage and I did my first prying at a triple clamp.

Wow, I never thought a cast aluminum triple clamp would be so easy to bend!

I put one side of the lower clamp in the mill vise, oriented the clamp so the two holes for the stanchions were vertical (so I could try and pull only in a horizontal plane so I would put as little extra twist into the clamps as possible) and then put a fork leg in the other hole and clamped it up. With an 18-24" lever arm it is not a question of "brace yourself and pull", but rather a "I wasn't even pulling hard yet and I hope I didn't overbend things."

In any event, the front axle is now almost parallel to a line across the fork tubes. It is about .020" high on one end.

That's not at all perfect, but the fork tubes are almost dead on in the same plane now (a couple thou off over 3" and I've got to measure to see if the axle is out instead of having it glaringly obvious to a visual inspection.

Based on how easy these parts have been to bend my guess is that a very sizeable majority of old dirt bikes that are in use today are probably bent more than just a little bit.

Before I go back to fiddling with wheel lacing/truing/alignment I'm going to disassemble the front wheel, clean the remains of the hub paint off and lube up all the spoke nipples so they'll have a chance of resisting corrosion in the future.

It will be interesting to see how much difference this all makes to the offset of the rear rim.

(The East Coast) Michael Green sent me some detail photos of his Miller Highboy framed Sherpa T, and a scan of a period Miller catalog page showing the aluminum mufflers that were available then:

I discovered that the OD of the VM26 Mikuni bellmouth is just a bit less than the nominal (unstretched) ID of the Miller air filter boot. I made a small adapter to press onto the bellmouth to increase the OD to the point that the boot has to be stretched to fit.


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© 1996-2008 Michael Moore, last update for this page 19 October 2008

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