Thursday, March 13, 2014

Derbi T.A.T. Build start and Derbi Piston port differences.

Soooo if you havent heard, a bunch of us are doing the Trans American Trail on Derbis.  The idea was thought up by Blk Blk Shawn L and its a pretty rad concept. The trail spans from Tennessee to Oregon and its mostly off road. Check out the site Here! and like or donate if you'd like, anything helps. Lots of rad dudes are working together to make this happen! This isnt limited only to Derbis but we thought it would be rad to all ride the same bikes and to coordinate tools and parts between us. If you want to join, that would be rad! 

So I tracked down a 1979 Derbi T.T. for my T.A.T. bike. It was completely seized and pretty shitty when I got it. What a better bike to destroy....right? 

Ive been doing some thinking and I can only afford to take a couple weeks off at the shop. So Im planning on just doing the trail up to Denver so the shop doesnt get too far behind. With all the discussion between us planning on going there are some very modified bikes in the works. My bike is going to be sort of an experiment/test of the stock components of a Derbi.  I am planning on riding a completely stock Derbi other than pipe, gearing, and frame mods. That means 12:12 Dellorto sha, stock top end, all the goodies! I am such a fan of stock bikes and I really feel like gas mileage is going to play a big part in this ride. The T.T. has a stock 1 gallon tank and I recently purchased a sweet Acerbis front fork tank that carries 1.3 gallons ($40 shipped ebay!). 

I need to convert the Acerbis tank from siphon fed to having a petcock, but that shouldnt be too hard.  So 2.3 gallons and a 12:12 should get me a ways down the trail. I just worry about people using kits and large carbs because we all know how off and on throttle kills mileage.  

Im rebuilding the engine right now and trying to salvage any stock parts possible. Got lots of ideas about this bike! Everyone is coming up with super rad ideas for their bikes, its inspiring.  Anyways stay tuned for some dumb offroad stock Derbi updates.


Ive been wanting to posts pics of Derbi PP differences for a while. I finally have 2 piston port engines off bikes and next to each other.  As you may know some Derbis (Laguna Sport / RD-50 / C-5 Diablo) came with tubular cradle frames and some came with stamped step through frames (SLE / T.T. / SL). All of these models came with either piston port or pyramid reed engines depending on the year.
 Well the engines that came on the tube framed models have a few differences than the ones that came on the step through frames. The differences were for clearance and mounting issues, check these pics. 

Step through frame
 Engine uses 3 mounts, one top front, one top rear, and one bottom rear, just like how most traditional moped engines bolt up to a frame.
Tubular frame
Uses 3 mounts as well. one top rear, one bottom rear, and one bottom front. It's hard to see the bottom front with this photo. See how the top front mount isnt used and doesnt even come with motor mount bushing in it. If you used this engine on a step through frame, you would have to figure out a solution for the front motor mount bushing being absent. Also note how close the head fins are to the frame.
 Left: Step through PP engine. Right: Tube frame PP engine.
Look at the engine studs on this tubular frame style PP engine. They only stick 1 and 3/4 inches out of the case compared to the 4 inches the engine studs stick out of the step through style PP engine. It is designed this way so you can remove the head and cylinder with very little clearance room of the tubular frame. Food for thought for anyone experiencing clearance issues on a custom build. Also note the 2 bolts under the crank case, those are the front engine mounts for this engine. 
Left: Step through style PP cylinder. Right: Tubular frame style PP cylinder. 
 The reason the right cylinder has large holes is because the engine nuts are 3 inches long because of clearance issues with the frame. 
 Tubular frame style Derbi head.  This is one of the 3 inch deep engine nuts for holding the head and cylinder on.  Dont lose these!!!!! Important!!! Notice the large holes for the large nut. 
 Traditional step through frame style PP head. <--- long title. Regular sized holes for engine studs. 

So if you have a random Derbi engine or are doing an engine swap, hopefully this can help you identify you engine or plan ahead for a build. Freaks.

Below is a random Derbi discovery. A stock v1 ring works as the bottom ring on a stock Derbi piston.  Who knew?!


  1. Informative. I also specifically like that one picture of the TT. That one looks responsible.

    1. I try to use all my own pics but yours is the best pic, gotta pay homage to iconic yellow Derbi T.T.s of google.

  2. Glad you are making use of my mostly dormant but once wonderful moped blog.

  3. ahhh so thats why those big holes are in some of those heads.

    those big holes is what gave me the idea for the hobbit head conversion with spacer bushings. i just happened to have one laying around.

    nice to see so much blogging as of late, i've been trying to keep up on out of control better too! retro blogging resurgence, just like 2009!

  4. also,

    some of the guys on ADV rider complain about that acerbis tank cracking at the seam. better look into that. one guy said he went through 3 or 4 of them and acerbis kept replacing them, until he finally just gave up.