The Macias Tribsa
THE MACIAS TRIBSA
A Triumph Lover’s Answer To BSA’s 500 MX
JODY NICHOLAS
The names Macias and Triumph have been synonymous for almost 25 years. As far back as 1948 Danny Macias was a Triumph dealer in Orange County, California. Danny was an active competitor for the four years he owned the shop, but began riding more competition events after he sold it. One year he was high-point rider in TT competition at the now-defunct Console Springs race track, to the east of Los Angeles.
Drag racing, Bonneville salt flats and track racing challenged Danny for many years, and as a master mechanic at Bellflower Cycle Shop he was able to build and tune many highly successful Triumph competition bikes. His two most recent riders are TT champion Skip Van Leeuwen and winner of the 1971 8-mile national at Ascot, Tom Rockwood.
Shortly before the start of the 1971 AMA racing season, Danny was put in charge of the racing departments of the BSA/Triumph combine. The results of his efforts are now history: BSA and Triumph riders took six out of the first ten finishing positions in the AMA national championship series including Dick Mann’s grand national championship victory, and runners-up Gene Romero, Jim Rice and Dave Aldana.
Being a former compeiiiion rider, Danny enjoys a good “cow-trading” session every now and then as well as an occasional trip to Mexico (Baja, California) with members of the infamous Gold Star and Antique Four-Stroke Club. In fact, Danny’s previous 40and 30.5-cu. in. Triumphs have proven to be among the most powerful and reliable bikes in the group.
WHAT DOES A Triumph lover do when BSA (Triumph's sis ter company) builds the most sophisticated and best handling "big banger" motocross four-stroke available today? It's not that Danny Macias doesn't like a Single, he just prefers a Twin; so he grafted a 30.5-cu. in. Triumph Tiger 100 engine into a BSA B5OMX chassis and has come up with what must be one of the best handling Triumph 500s in captivity!
Shoehorning the Tiger 100 engine into the BSA frame was a relatively easy job, requiring only the making of special aluminum engine mounting plates. The only other special bits needed for the swap were upswept exhaust pipes that originated on a 500cc Triumph but had to be altered slightly to fit properly. All else on the bike is standard with the exception of the handlebars, the Triumph decal on the gas tank and the progressively wound rear shock springs.
A close look at the Triumph engine won’t tell you much about what’s inside, but the absence of oil leaks after a hard day’s running reflects Danny’s careful assembly technique. In fact, the only clues about the inside are the 32mm Amal concentric carburetor, a special rubber intake manifold spacer block and two non-standard engine breathers. One is in the crankcase behind the cylinders and one exits at the clutch case oil filler plug.
Modifications to the engine have been made not only to give slightly increased performance but also to make the engine more reliable and smoother running. Although a 32mm carburetor is fitted, the actual size of the inlet ports was left unchanged at a nominal 28mm. Inlet valves with a head diameter approximately 1/16-in. larger were fitted, but only because they are made of better steel than the standard items. A set of JRS-1 valve springs with aluminum valve spring retainers, in conjunction with special alloy pushrods, allow high engine rpm without fear of floating the valves.
The standard compression ratio of 9:1 has been lowered to the region of 8:1 for increased engine smoothness and to make the engine happier running on the low octane gas which is abundant in Mexico. A standard camshaft is used. When Danny built the engine the only crankshaft he had around was one he lightened for use in a 500 Triumph road racer several years ago. This makes throttle response a bit quick, but plans to install a standard crank and flywheel have been made.
No changes were made to the transmission or the clutch as both are reliable, but the scarcity of rear sprockets means changing the countershaft sprocket to alter gearing which is too low at present.
Engine weight is less than 20 lb. heavier than a BSA B50MX engine, and slightly over half of that increase is placed on the front wheel. This makes the bike somewhat easier to “steer” than a B50MX. Power is a little sudden with the light flywheel, but is helpful in breaking the rear end loose and into a slide if you feel like practicing half-mile racing.
In order to make the exhaust quiet (and the bike “forest legal”) a spark arrester/muffler was fitted. Very little top-end horsepower has been lost and mid-range power delivery is slightly stronger and decidedly smoother. There is no feeling of “caminess” in the rev-range, making the bike very predictable and easy to ride.
Word just received here at the Cycle World offices has it that similar machines will be available from local Triumph dealers in the future. Yes, Virginia, there is something distinctly exciting about a Twin!
