05 September 2011

Dirt bike project update...fun with cylinder heads

I've been working on the Atom Bomb/RaceTech Suspension dirt bike project a bit evenings and weekends. Since this is a long weekend I decided to figure out what to do about the stock splayed port carb setup, since each carb stuck out roughly 4" past where my knees will be when riding this thing in anger.Not to mention, I have to put an exhaust collector back there somewhere.

I very nearly went to the dark side and put a single carb TR7 head on this bike. Luckily they're IMPOSSIBLE to find (even though I've had like three TR7s through here in the past year). I was so focused on a single carb head that I almost modifed a late TR6 head and used that. Then I smacked myself in the face and realized how lame it'd be to take the easy route. Back to my original plan of making a twin carb setup take up less space.

After determining that simply making manifolds that pointed straight back wasn't gonna cut it, I decided to get serious and make a parallell port head out of the stock T140 splayed port deal, which looks like such (all these pics get bigger if you click them):

First step was mill the flanges as flat as possible and fill the mounting holes, which would be useless to me:

Then I welded all that mess up enough to get a flat surface, and milled it again:

Since we're also moving the ports inboard, I determined that I needed to add some more material to build the pad up. Did that, then milled flat once again. Once I had my flat mounting base, I sort of roughed in the ports (very rough) so I could locate my manifolds.

All marked up and mounting holes drilled:

With that out of the way it was time to shape the outsides of the new flanges and the ports. Here's what the new and improved head looks like all done. It was a TON of work to not only get the ports lined up and sized properly, but make it all look more or less like it came that way. Pay no attention to the valve guides, I knocked them flat on top so I could clamp the head on the mill. I didn't want to put the head through all these heat cycles with no guides in it.

In this next picture you can see that I left some weld visible on the inboard side of the flanges. Got lazy. You CANNOT see it when the head is on the bike, so I'ma let it fly. You'll also notice that I had to weld probably 1 1/2" down into the ports to add enough material to shift everything. Not fun, don't recommend it. But it did allow me to carry the port in dead straight and the right size/shape.

Here's what the whole deal looks like on the bike. Carbs are tucked in nice and tight!


  1. holy shit.
    nice work.
    is carbs sticking out at an angle that big of an issue? is that how the metisse triumphs were back in the day? McQuueeen style?

  2. Superbly written article, if only all bloggers offered the same content as you, the internet would be a far better place.. Best Dirt Bikes

  3. Really a very fine way to express and looking a very pleasant article with full of healthy information. I am definitely going to tell in my class about this platform and the information shared here. Very well written.
    Motorbike adventures

  4. Thanks for sharing great information, if you need more information about custom motorcycles visit now.
    custom motorcycles