High rpm 125 motor

I'm new to discussion forums. building what I'm calling a Super B, and I want to build a high RPM engine for it. Any advice? Hardened seats? Why is the compression so low? Will D15 pistons work on it? Looking for about 3,000 Rpm. Bigger valves? I'm building my own exhaust and intake out of stainless steel.

I have no idea what your intentions are for using this tractor, but give these items consideration if you would. I have a WD-45 that I take on "tractor rides", and some days they might be 75 miles in length, which means 5 to 6 hours of engine running time, depending on the pace.  Some days, I am near full throttle all day long. I have zero model "B" engine experience, but a fair amount of D-14/D15 experience. I cannot imagine 3,000 RPM continuously on a tractor ride with splash lube connecting rod bearings. Maybe they'd be fine, but the correct choice of engine oil viscosity would be very important.  A pressurized oiling system like the D-14/D-15 have would surely be better.  A D-15 series 2 governor spring would get you 2,400 RPM if your governor linkage is the later design "B" parts.  An additional helper spring on the carb side would get you your desired 3,000 RPM's. Keeping the governor action like a stock tractor would be important I think. A slightly larger carb from a D-14 or D-15 series 2 would help add RPM and HP as well.  You don't need bigger valves. The bigger carb will do more than bigger valves and a stock carb. More compression would be good for more HP and torque at all engine speeds. Good luck with your "One-of-a-kind" project.

Thanks for the well wishs.will the d15 motor go right in? Would the shorter stoke 125 put up with the higher rpms?

Super B wrote: Better

know anybody that has ad15 motor for sale?

Just a novice at any engine performance mods here, so take this for what it is worth.

A "CA" rear end with 50 inch tires @ 1650 RPM should do 13 MPH. So, at 3300 RPM it would do 26 MPH.  That's plenty fast enough for any tractor ride I've ever been on !!!   At 2400 RPM (D-15 governor top speed) it would be rolling along at 19 MPH approximately.  I wouldn't even consider a 5 hour tractor operating the engine speed with my foot.

Super B,

Pretty small cam shaft, pointy lopes. all for low end. springs are pretty light. If you just want rpm it might be ok, but won't be much power.

I have many different sizes of carbs lucky and i know that I won't get it the first time thanks for the input

I have many different sizes of carbs lucky and i know that I won't get it the first time thanks for the input

Wait till Mr Bill here's about air conditioning on a B and how fast you want to go just curious

30 mph would be great i wouldn't drive it that fast normally but it would be nice to know it could

My gas MF-135 and gas 4020 without any mods used to run at 22 or 23 mph on the road. Most tractor rides around here plan on 12 or 13 mph.

Gerald J.

I built my pulling Allis C to run 4000 RPM. I had the engine shop grind a D 15 crankshaft to a 4 inch stroke and balance the crankshaft and flywheel. I sent the D 15 camshaft to Crane Cam to grind it to the engine set up. The cross drilling of the crankshaft was plugged and redrilled to have a oil pressure system. I used small block Chevy valves and ported the head. I took .150 off the head. I modified the pistons to enter the firing chamber and weighed each one on a gram scale to get them at the exact weight each with rod and bearings. The head looked like the last one I did in the pictures below.