Seriously considering a Phatmoto

[Gordy]

I've been looking at the phatmoto bikes, and for 700 dollars on sale that's a lot of bike. I've wanted to put a Briggs flathead on one for awhile too. Maybe when they go on sale next year I'll pick up one and build a killer bike out of it.

I've been looking into building a 3hp Briggs for a bit more HP than 3 (aiming for 8 but 6 if I'm lucky lol) and that would make a killer power plant for one of these. The only Issue I've found is no one makes a connecting rod for the 3HP out of billet aluminum. I looked into it and the rod itself can stand quite a bit of rpm, but the steel oil slinger on the end is the issue. It tends to snap off between 4000 and 5000rpm. Perhaps if this could be replaced with something stronger the stock rod would maybe hold up? What do you think?

 

[cloakedvillain]

How many half finished projects do you have right now? I have 2. And that's 2 too many!

 

[Gordy]

How many half finished projects do you have right now? I have 2. And that's 2 too many!

Just my Phantom 85 bike which I have all the parts for, and my trail 90. The trail 90 is priority #1 as far as big projects, but I hope to have it finished (or at least riding) by mid October. That's why I said next year when it goes on sale lol.

 

[michael whiteman]

Back in the early days of hot rodding Gordy we did a lot with stock cast rods before the aluminum ones became reasonably priced. First you polish the sides with a grinder and finish up with a fine grit sanding belt. This reduces the chance of a stress fracture happening here. I remember honing the small end to facitilate floating wrist pins along with drilling an oil passage into that area. The final step was to shot peen the rod which relieves the tension and neutralizes the molecules on the surface.

I haven't seen a B&S rod but if the slinger tends to break off I would just remove it right away. I would fabricate one from scratch out of steel and mount it to the rod using longer rod bolts if necessary.

I feel that valve float will be a bigger issue in a B&S build. Besides, good air flow into a flathead style engine has always been the weak point. I know you'll make more power that you'll need, so don't worry about the rod if you follow my advise. My 302 Chevy had the rods preped like this and I would turn it to 8K rpm with it's roller cam. Ill' bet there's even a YouTube video on improving the cast connecting rods. Let me know when you get ready to do this.

 

[Gordy]

Back in the early days of hot rodding Gordy we did a lot with stock cast rods before the aluminum ones became reasonably priced. First you polish the sides with a grinder and finish up with a fine grit sanding belt. This reduces the chance of a stress fracture happening here. I remember honing the small end to facitilate floating wrist pins along with drilling an oil passage into that area. The final step was to shot peen the rod which relieves the tension and neutralizes the molecules on the surface.

I haven't seen a B&S rod but if the slinger tends to break off I would just remove it right away. I would fabricate one from scratch out of steel and mount it to the rod using longer rod bolts if necessary.

I feel that valve float will be a bigger issue in a B&S build. Besides, good air flow into a flathead style engine has always been the weak point. I know you'll make more power that you'll need, so don't worry about the rod if you follow my advise. My 302 Chevy had the rods preped like this and I would turn it to 8K rpm with it's roller cam. Ill' bet there's even a YouTube video on improving the cast connecting rods. Let me know when you get ready to do this.

Thanks! I already planned on polishing the rod. I was thinking of maybe making an oil slinger with nylon or something else that is a little flexible. I do realize finding some stiffer springs might be a challenge and that fuel and air flow isn't great. I do plan to clean up the casting in the ports and shave the "eyebrows" inside the block for better flow. I was also looking at the stock head and thought maybe I could make my own head with a chunk of aluminum mill stock. It's a pretty simple design, and I could change the shape of the combustion chamber a bit to allow for some better flow. I might start this project pretty soon because all I really need to buy that has a big price tag is the flywheel, however that comes after all the preparation of the inside.

 

[michael whiteman]

Interesting thought ...........nylon slinger. Personally I would not want it to flex. It will be flexing thousands of times per minute, and at what point is it going to break off. I wouldn't even trust aluminum for the long term.

The eyebrow mod is worthwhile, although it does lower the compression ratio. I think finding stronger valve springs would not be too difficult. Make friends with someone in an auto parts store who will let you look through everything he has on the shelves. You can always use shims to increase the pressure. Just be aware of spring stack.

Do B&S flywheels have a history of blowing up like my Predator? You might not have an issue if the magnet is fastened better. Show us a picture sometime. What rpms are you planning to run?

I don't see anyway to make a cylinder head without a milling machine. I think the fins would stop you cold. With the flat top piston you could mill the head down till the valves hit the head and then with a Dremel grind the pockets in deeper to regain the clearance. It's going to be interesting following you along on this build

 

[Gordy]

Interesting thought ...........nylon slinger. Personally I would not want it to flex. It will be flexing thousands of times per minute, and at what point is it going to break off. I wouldn't even trust aluminum for the long term.

The eyebrow mod is worthwhile, although it does lower the compression ratio. I think finding stronger valve springs would not be too difficult. Make friends with someone in an auto parts store who will let you look through everything he has on the shelves. You can always use shims to increase the pressure. Just be aware of spring stack.

Do B&S flywheels have a history of blowing up like my Predator? You might not have an issue if the magnet is fastened better. Show us a picture sometime. What rpms are you planning to run?

I don't see anyway to make a cylinder head without a milling machine. I think the fins would stop you cold. With the flat top piston you could mill the head down till the valves hit the head and then with a Dremel grind the pockets in deeper to regain the clearance. It's going to be interesting following you along on this build

As far as I know they aren't as prone to blowing up, but I can get a 3hp aluminum flywheel for it and that is a part I'm willing to splurge on. I was thinking I could use a chop saw with the depth set and a fine tooth blade to cut fins in the aluminum. It would make for an interesting experiment, especially if I can get some cheap aluminum mill stock. I'll post some pictures of the 2 culprits I have soon. I was aiming for 10,000 rpm, but considering the limited availability of performance parts 6,000 - 8,000is probably more reasonable. But who knows, the sky's the limit!

 

[michael whiteman]

You're an " out of the box" thinker for sure Gordy. I like that about you. I would take that metal cutting blade and put it in a table saw. I think the cuts would be more uniform. Cool idea! If you lighten up the piston, rod and aluminum flywheel I think 7000rpm might be doable. You'd definitely want to dynamically balance this complete assembly. There are places that will do this for you.

Did you ever consider making a supercharger out of an old 1970's GM smog pump?

 

[Gordy]

You're an " out of the box" thinker for sure Gordy. I like that about you. I would take that metal cutting blade and put it in a table saw. I think the cuts would be more uniform. Cool idea! If you lighten up the piston, rod and aluminum flywheel I think 7000rpm might be doable. You'd definitely want to dynamically balance this complete assembly. There are places that will do this for you.

Did you ever consider making a supercharger out of an old 1970's GM smog pump?

I've thought about it, but I think going naturally aspirated is a lighter and smaller package for a bicycle. If I do build another one I might just for fun. Now that I've looked into it, the 3hp aluminum flywheel would fit a 3.5hp engine which has a larger bore and stroke. I could start with that engine, which is just a bit taller than the 3hp, and be able to squeeze a bit more out of it due to a larger displacement. Plus the 3.5hp I have is in a lot better shape than either of my 3hp engines. I might pick up one of those smog pumps however, our real estate agent has taken a liking to us and her dad wants to take me to the junkyard with him some time, so I might just snag one then. Great idea!

 

[Chainlube]

You're an " out of the box" thinker for sure Gordy. I like that about you. I would take that metal cutting blade and put it in a table saw. I think the cuts would be more uniform. Cool idea! If you lighten up the piston, rod and aluminum flywheel I think 7000rpm might be doable. You'd definitely want to dynamically balance this complete assembly. There are places that will do this for you.

Did you ever consider making a supercharger out of an old 1970's GM smog pump?

Woodworking tools work better for aluminum than metal working tools. I've cut lots of pieces on my chop saw and table saw.

Cut the shape and drill the holes first, then rip the fins in a box clamp.

Check out Warped Perception on YouTube for the clear head trials.