372xp Big Bore w/ Tillotson carb (a true 80cc)

Hyway 372xp big bore cylinder which would be for a Husqvarna 372xp chainsaw (52mm x 38mm). The crank bought by Don Butler initially claimed to be balanced to 52%, turned out to be only 35%. To rectify this, I bored out the original holes and drilled them deeper, achieving 50% balance. The reciprocating mass was 156g, and I removed 23.4g to reach 50% balance.

I inserted brass bushings (10x12x8) in the piston to accommodate a 10mm wrist pin so I could still use a wrist pin needle bearing and not a 12mm wrist pin and a bushing on the conrod instead. The case is an undrilled YD100 case (super special), and I modified the transfers because the original transfers are horrific. The 66cc/69cc has better transfers. Additionally, I added an M3 to M5 fuel fitting to run the impulse line on a tillotson carb.

After building an MS440 last year, I've developed a preference for hybrids over "china dolls." I'm excited about this build, especially with my first experience with diaphragm carbs, which offer easy jet adjustment and can run in any position without flooding.

For those interested, here are the port timing and durations for the Hyway 372xp big bore cylinder on a 38.5mm stroke with a long rod.



Exhaust - 94° (172°)
Trans - 116° ATDC (128°)
Intake - 73° BTDC (146°)
Blowdown - 24°

 

[robe]

That is nice. Did you make or buy the adapter plate? How thick is it?

 

[Nickt919]

Nice engine build. I’d be thinking about the clutch holding when it hits the pipe.

“The crank bought by Don Butler initially claimed to be balanced to 52%, turned out to be only 35%”

When a vendor advertises something like a crank being balanced to a specific % BF, it’s an estimate in a sense.
It was probably done up with a 47mm piston..Minarelli type build in mind.
The saw conversions are less common but gaining in popularity fast.

 

[ImpulseRocket]

50-52% is about all most people will get for BF wih a 52mm saw piston. Same is true for the Phantom 85. I managed to get 54 with my meteor piston, but I made all of the holes as big as I could, and aside from the center hole drilled at the crank pin, all of them were fully drilled through, and my outer 2 holes were about as close as I could get them. 50% is actually a good enough figure to eliminate a majority of the primary vibrations out of the operating RPM range, unless you plan to regularly go very high into the RPM range.

 

[cloakedvillain]

What about adding weight to help balance with balance? About 1:30 into the video he starts talking about it.

 

[ImpulseRocket]

What about adding weight to help balance with balance? About 1:30 into the video he starts talking about it.

Yes, you can do that too. Tungsten plugs are a very common method of adding weight to a crank. Just takes proper machining for tight tolerances on the plug so it can't just come out (press fit) and the right math to determine how much weight is needed.

 

[cloakedvillain]

Very rambley but Matt here(dirtygarageguy) suggests drilling and tapping holes, then using lead to fill it so that the weight can be tuned by drilling it out.

No precision machining, just a drill press and a tap handle.

 

[ImpulseRocket]

Very rambley but Matt here(dirtygarageguy) suggests drilling and tapping holes, then using lead to fill it so that the weight can be tuned by drilling it out.

No precision machining, just a drill press and a tap handle.

The problem with thread retention is that the plug can potentially back out of those threads. A proper interference press fit is much more secure.

One could simply start by adding weight to the "bottom" side first, and then drill out the "top" as needed.

 

[francisjohn]

Nice engine build. I’d be thinking about the clutch holding when it hits the pipe.

“The crank bought by Don Butler initially claimed to be balanced to 52%, turned out to be only 35%”

When a vendor advertises something like a crank being balanced to a specific % BF, it’s an estimate in a sense.
It was probably done up with a 47mm piston..Minarelli type build in mind.
The saw conversions are less common but gaining in popularity fast.

I didn't want to bore everyone with the specs, but here we are.

Upon speaking with him, he said it was balanced to 52%, I then asked for what top end as I know the reciprocating masses would vary. The 47mm top end is 103g (we can do +/- 3g for good measure) and let's use 37g for the conrod, so 140g total. When I received the crank, I first calculated the balance factor and used the 47mm top end in my equations, even tweaking the numbers on the conrod and the top end, ultimately either A) He didn't know what was done or B) Mis-calculated severely (the mis calculation is so severe that it's unrealistic)

I knew the 372xp big bore recip mass would be more so I didn't take the specs for a 47mm, apply it to the 372xp and say it's inaccurate, that would be wrong. I fully anticipated needing to remove more mass assuming it was 52% and with a lower balance factor once the 372xp specs are plugged in. To be fair, I didn't have to drill a whole lot and most the work was done so I'm not too upset.

50-52% is about all most people will get for BF wih a 52mm saw piston. Same is true for the Phantom 85. I managed to get 54 with my meteor piston, but I made all of the holes as big as I could, and aside from the center hole drilled at the crank pin, all of them were fully drilled through, and my outer 2 holes were about as close as I could get them. 50% is actually a good enough figure to eliminate a majority of the primary vibrations out of the operating RPM range, unless you plan to regularly go very high into the RPM range.

I noticed something odd in the MB community where everyone says 55% is the best, everyone wants 55% but nobody can say why they're saying 55% other than it's what they heard from others, who also repeat it and don't know why. I don't ride at higher RPM (more than 8k) I would MUCH rather have better low end-mid than top end, I also live in a city and it's constant stop and go


Regarding making the holes as big as you can, I've seen it done where people use 1/2" bits and it's calling it close as theres only 13mm of material on the web to drill. I would rather use a smaller diameter and drill deeper than use a larger diameter and drill less.

My MS440 crank has the center holes at 16.5mm deep (it's a thick 17mm crank) and the side holes are around 8mm deep and it's to 55%, unlike this crank where I had to drill much deeper and a larger diameter. Crank balancing is an individualized approach based on what the current factor is and where you want to get to. I would never buy from a seller who just drills the standard x3 holes on each side and calls it a day, however people buy it not understanding what a balanced crank is and think "holes = balanced)

Very rambley but Matt here(dirtygarageguy) suggests drilling and tapping holes, then using lead to fill it so that the weight can be tuned by drilling it out.

No precision machining, just a drill press and a tap handle.

I've seen RDM cranks where that is done, my only concern as @ImpulseRocket has mentioned that the hardware would back out, and then what?