Motor Mounting on Trek 3700

speedster239 · #1 12-07-2009, 11:28 AM

Hey guys,
I finished installing my new F80 kit from thatsdax.com on my Trek3700 the other day. It turns out that the down tube and seat tube were both too large for the motor mounts to fit onto.

With the down tube, the bolts wouldn't even extend past the tube itself, so using the three hole adapter the normal way wasn't even an option... I simply placed a nut on the exposed bolts coming from the block, then the three hole adapter, and then nuts to hold the adapter in place right near the end of the bolt. Luckily, I didn't need to drill as there was a hole available for the water bottle holder screw. I unscrewed this screw, lined up this screw with the hole in the frame and the hole in the center of the adapter and tightened! Talk about a perfect fit! After this, I went ahead and mounted on the seat tube.

The mount for the seat tube also didn't fit and the two bolts barely barely squeezed past the frame. Luckily I was able to squish them past the tube and I let the end of the block itself just rest on the tube. I placed both the clamp for this application, and the unused clamp for the down tube onto the two bolts, in addition to 7 washers on each bolt for space (as the threading on the bolt ended before the nut would even reach the clamps) and I tightened...

Snug fit!

Downside is that the carburetor barely squeezed in, even without the air filter on. So, instead of using the air filter assembly, I just placed the piece of cloth in front of the intake. I have no other option, the carburetor is literally right up against the tube. I'm in California, so it doesn't rain much here and this won't be too much of an issue, however I'm wondering how I can at least protect the topside from water.

wheelbender6 · #2 12-07-2009, 02:26 PM

For engine mounts, buy some muffler clamps that are close to the diameter of your frame tubes. Drill holes in the flat part of the muffler clamp that match your engine mount bolts. (You can also buy engine mounts for large frame tubes from some of the vendors on this site)
For the carb, saw your intake manifold in half. Connect the two halves with a heater hose and clamps. You intake is now flexible and you can use the air cleaner again by allowing the carb to fit against the right side of the seat tube.
There are many posts with pics of these remedies. Just search and browse.

NerpAngel · #3 01-23-2010, 03:03 PM

Can you post some pics? I think I have a very similar bike with the same problem and I'm not sure exactly how you mounted it

Cabinfever1977 · #4 01-25-2010, 05:27 PM

Do not use a bottle holder screw to hold a mount(unless you drill the hole threw and use a bolt), there is other ways like useing clamps.

Ghost0 · #5 01-26-2010, 10:15 AM

Here is a photo of the front mount we sell. Note the offset of the bracket allows the engine to sit as low to the frame as possible and still allow for the over sized tube. If you have the means I am sure you can fab something similar.

NerpAngel · #6 01-31-2010, 08:15 PM

Where can I find those pieces of metal that attach the U bolt to the motor? I couldn't find them today at my Ace Hardware store so maybe it's a custom piece?

Ghost0 · #7 02-01-2010, 01:29 AM

Here is a link.
Sick Bike Parts

NerpAngel · #8 02-01-2010, 04:03 PM

Oh wow. I can't believe I passed over that when I was searching that site!! Thanks Ghost0!
Judging by your picture I think we have the same bike, so after looking at the sizes available for the mount, you probably got the 1 1/2 down tube width and the 1.9 inch engine stud right? I measured mine and it seems like that would be the combo.

Ghost0 · #9 02-01-2010, 04:19 PM

Actually the bike in the photo I posted is a 2 inch frame and the engine has 1.6 inch stud spacing. Please measure yours before ordering.

NerpAngel · #10 02-01-2010, 04:22 PM

Well wait a minute I'm confused... because the front of my motor will need to sit on a tube that's around 2 inches whereas the seat tube is only 1 1/2...