Balancing?

#21
Your method is correct but I do not understand why you need so little weight to balance. I guess it is possible to use a 39% balance factor it just reduces vibration in the horizontal direction (relative to cylinder axis, and increases vertical vibration. I do not know how free the rod is on the crank, in the video above its a roller bearing crank with a lot less friction than your setup. You may want to slowly add weight till it stops any place you put the crank then add small washers one at a time till it starts going down and it no longer stays where you put it and if it is more than a washer or two then you have a friction issue causing inaccurate results.
 
#22
Yes, Only 59 grams extra (with the rod) seems low to me. Make sure your leveling instrument is level and has no friction. Measure it both ways.
And what ole4 said; compare the weights of stock and parts you're running will tell you what "happened". I still have to do that at mine. Today new crank bearings arrived, so I'll be in it soon, hopefully.
New crank bearings? Yes, mine have 0,1 mm play (0.004") up and down. I can't find any service limit on that, but I find it to much.
 

noseoil

Active Member
#23
Still need to tear into the "old" motor to see what the parts weigh. Popeye, will you be able to get some weights on your motor? I'd be interested to see what the parts are in it. Which motor is it on the bike (the model number)?
 
#24
Popeye, will you be able to get some weights on your motor? I'd be interested to see what the parts are in it. Which motor is it on the bike (the model number)?
It is a Honda GX160. Type:QMDS It has now an aftermarket crank, stock flat top piston (ZH8), ARC rod 3.303. ARC ultralightweight flywheel.
When I replace the bearings, I was planning to weigh the parts en compare them to the stock parts. Also like to weigh the piston, as I have an aftermarket piston, which might be lighter.
 

noseoil

Active Member
#25
Here are the numbers for the ARC rod & Bullfrog piston. Piston is 253g (50 grams heavier than the Predator standard flat top hemi piston at 201g) Arc rod is 149.5g (not much heavier than the 142.3g Predator rod). There's 55 g more with the aftermarket combination. The engine was in pretty good shape on the bottom end. Crank was good but still out of round. End play had gone from 0.010" to 0.014" after the high speed runs, so everything was good. Block was fine.

I cleaned up the piston, scraped off the carbon, ground out the plug ding & then it's ready to go back together. Just need the bearings , SS valves & then I'll hang the new crank & use new gaskets. Decided not to monkey with the balance on this build, not worth the trouble, after seeing how well it's held up to the runs it's made. A bit more detail on the site if you're interested.
Piston-3.jpg
 
#26
Here are the numbers for the ARC rod & Bullfrog piston. Piston is 253g (50 grams heavier than the Predator standard flat top hemi piston at 201g) Arc rod is 149.5g (not much heavier than the 142.3g Predator rod). There's 55 g more with the aftermarket combination. The engine was in pretty good shape on the bottom end. Crank was good but still out of round. End play had gone from 0.010" to 0.014" after the high speed runs, so everything was good. Block was fine.

I cleaned up the piston, scraped off the carbon, ground out the plug ding & then it's ready to go back together. Just need the bearings , SS valves & then I'll hang the new crank & use new gaskets. Decided not to monkey with the balance on this build, not worth the trouble, after seeing how well it's held up to the runs it's made. A bit more detail on the site if you're interested.
View attachment 245433
Site? Where?
Yes. The arc rod are almost the same weight. Mine has nearly the same small end weight. Big end is a bit heavier because of the bearings.
Your balance factor comes down to around 30% with the piston.
 

noseoil

Active Member
#28
It's says "Arias" inside the bottom of the dome. After looking it up online, looks like Bullfrog got an Arias forged piston, then had it machined to their specs for a match to the Predator hemi engine. I think that's why this thing shakes so badly at the wrong rpm range, it's slinging so much weight it has to show up somewhere. Glad I bought mine 2 years ago, they're up in price & back-ordered now, but it's a good way to increase the compression with a stock head. That's why I tore things apart & looked at the bottom end. Pretty tough piston though, cleaned up OK.

Just waiting for bearings & a new fuel tank now. Getting rid of the stock tank & replacing it with a frame mounted 3x8 spun aluminum one from Sandstorm. A bit less weight to lug around & a better gravity flow with the higher placement of the tank.

https://www.bullfrogperformanceproducts.com/product-p/200-1500.htm
 
#29
Can you take a pic of underside of it? Back in the day the MC forged pistons had a solid boss which was drilled for the wristpin. We used to use a ball end mill to mill a channel between wrist pin and bottom of piston. Depending on thickness of the dome could also remove some material underneath the dome. Ross pistons offers a lightened piston where they mill underneath.
 

noseoil

Active Member
#31
I guess I could set up a router, but it's not safe!Only other way I could do it would be with the drill press & an end mill & start playing, but I'm a wood worker by trade, so having me do this type of stuff might be an expensive way to make a mess. There's lots of meat in there, but the way I run this thing it's probably a good idea that it's extra thick & beefy. Really is a nicely made piston & someone went to a lot of thought & trouble to set it up & make them. Not sure about the back order status on them, might be a problem in the future... PopUpBot.jpg
 
#32
Looks nice but looking at the oil return drilling’s it looks thick. Can’t see the bottom of the skirt but can you measure skirt thickness? Is it the same all around or thicker 90 degrees to the wristpin? Just curious unless you have a mill and a piston vice or mill mounted chuck forget milling it. So adding some weight opposite crankpin to increase balance factor could help but in the end it’s not guaranteed. You could reassemble with stock piston and see if vibration is reduced. Would also show par difference in the two.
 

noseoil

Active Member
#36
Here's a shot of the bottom of the piston. I just went out to the shop again to look at the bottom of the pin on the edges which look to be fractures. It's just some residue from the stuff floating around the shop & oil, not stress fractures, as it looks at this magnification.

The other shot is a sketch of the dimensioning of the bottom side. Like I said, it's really a beefy piston & has a lot of extra metal, compared to a stock one, which looks pretty "weak" when you look at both. I don't think the engine will destroy the piston, if anything, the piston will destroy the motor. Maybe I should go to a billet block, head & some nitro to take advantage of this beast... P-Dim.jpg
P-Bot.jpg
 
#37
Billet blocks are very expensive but are pretty much indestructible. Even the thick cast blocks like Blockzilla can be welded up if it kicks out a rod.
Lots of guys weld up blocks at known weak points as insurance, not sure where those are on a clone but the info is out there.
 
#40
Today I have weighed 2 aftermarket pistons with piston pins. 1 piston is a flat top and the other is a dished one.
The flattop (chinese aftermarket) with Honda piston pin is 182 gr.
The dished piston with pin (both chinese aftermarket) is 233 gr.
The dished one is here because it was cheap and at the pictures it looked like a flat top piston. Well, I can use it to practice on with removing weight on the Bridgeport mill.
 

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