Last updated on December 30, 2018

Words and photos by Marc Biro

What do you get when you cross a Yamaha and a Penton? One potent ‘AHRMA legal’ Classic 125!

As a kid in high school, I was fortunate enough to own a brand new 125 Penton Six Days. At the same time, my best friend was the proud owner of a brand new Yamaha 125 Enduro. Come the weekend, my friend would have the lights stripped off his bike, and we would head out to one of our favorite So. Cal. riding parks, or the beautiful Mohave desert, and have a blast. Come Monday, he would have the lights back on his Yamaha, I would jump on the back, and off to school we went.

Fast forward to 2005. I got the bug to build a bike and go racing. AHRMA offers a 125 class that allows you to run 1971 and earlier 125cc machines. In this class, both the Yamaha and Penton are eligible. I was torn between which one I wanted to build. In the day, the Penton had the HP and handling, but the shifting was awful. The Yamaha was lightweight and shifted well, and by investing a few (maybe $75.00?) bucks, you had a motor that rivaled the Penton. The downside of the Yamaha lay in the handling department. What to do! Well….I just so happened to have a pretty good collection of both Penton and Yamaha parts. Out to the garage I went, digging through containers of Penton and Yamaha parts I had collected over the years. Before I knew it, there on my work bench was my creation. Before I go any further, I have to give it a name. Ok, let’s see, Yamaton? No. Pentama? No. Yamapen? No. Pentaha. Yes, I will call it the Pentaha! In fact, I will it name it the Pentaha 125 MB-001. It is the first one ever made. Right?

The mocked up bike was then disassembled, and it was time to really think this thing through. I pulled out some old Dirt Bike, Cycle News and MXA magazines, and started my search for Yamaha hop up articles. One in particular really caught my eye. It was the 1971 September issue of Dirt Bike. The cover stated: The world fastest 125! I quickly flipped to the article, and there it was, a 1971 125 Yamaha! Alright…24 reliable hp! Perfect, so the top end was done as per the article. I then found a gear set out of an AT-1 MX lower end, MX head, added a new rod and piston kit, all new bearings and seals, a new Mikuni, modified intake manifold, and an NOS high torque engineering pipe. With the motor complete, it was on to the chassis. It was a bit tricky sticking the motor in a Penton frame, but with my new welder and Makita cut off saw, it went as planned. A set of 34mm forks from a 1973 Yamaha 250 MX where installed into the stock but bored out Penton triple clamps, while on the rear a set of Girling shock were used.

Cosmetically, a MX Penton tank was used along with a set of 1976 RM fenders. The stock Penton airbox was kept intact, but was modified to accommodate the chain running on the LH side. A stock AT-1 conical rear hub was used along with a front conical hub from a 1976 YZ. Both where laced up to highly polished shouldered Akront rims. The frame was powdercoated black, while the airbox and tank were painted to match the RM fenders. Ok, done, done, done!

My first test ride down the street revealed what I was hoping for – one very fast Classic 125! Everything worked perfectly, and felt just right.