Active Member
Put the "new" motor back together again. Decided to say screw balancing with my tools & background, just run it & see how it holds up. New HF crank with new ARC bearings is a bit tight, so I'm dropping to 0w-20 oil. Hopefully it will work. It's back running again, so we'll see how long it lasts. No high rpm pulls yet, still letting it run in. I was surprised to see the oil was still good after a change & sending it to Blackstone Labs for analysis. Had extra lead, chrome & aluminum, but it was still a good 5w-30 after all the abuse, heat & hard pulls from the desert. Posted to the web site with what I did, so you can see how it looks.



Active Member
Been running a bit more with the new bottom end, changed 1st oil & did a couple of pulls to see how it runs yesterday. Had a slight tuning issue which was resolved. Added a little more fuel in the mid range by changing the needle 2 positions richer, much better transition now to WOT. It was slightly starved up here, go figure. Altitude is a good teacher for tuning. More confident in the SS valves not breaking also. Thinking the porting being different on this head may have contributed to the slight lean transition.

Fixed a fuel "sloppage" at the filler cap on the new tank. It's still shaking a lot at around 4,000 rpms & I was getting some gas on my junk as it shook itself through the rpm range & wound up past the vibrations with a full tank (new name for the bike may be the "$hit Shaker). Drilled out the cap, drilled a zerk fitting & added a hose to the center of the cap. No fuel spills now & no "junk" fires, I'm happy to report.

I'm running a final drive of 7.2:1 & it's pulling well enough at speed with the 19" tire. I've run it up to 7720 rpms now & it's strong (just a tad over 60 mph). I backed out of it at that point, still letting things run in a bit more before I run it up to 8000 again. I may change gearing to go a bit faster, but it's still a dog on the bottom end due to gearing, tire size & clutch. I may change things up again to a torque converter, but not any time soon (Polaris $$$). I'm just glad my "first" motor is still alive & performing well. Trying to decide about a new small block that doesn't shake as badly (non-hemi), or just building another bike for a 460cc & torque converter...
I would build a 460 for the bike you have now, unless you want to build the chassis differently for the 460.
And you may. If that’s where you're going to end up anyways, why put it off?
Wanted to add a bit more to this thread. I am building a 72mm tillotson block open motor. I wanted to keep the reciprocating weight as low as possible. I saw NR had a cast 72mm piston and ordered it thinking a casting will be lighter than a forging, but when I got it, it weighed in with clips, rings and pin at 262 grams, 62 more than the 200.6gr of the stock hemi piston complete. The wrist pin was longer and thicker. I tapered bored the pin and also milled some of the skirt when the pin is off to lighten it but it was still at 241.5 gr. I asked ARC about the wiesco 2.835 3 ring piston and he said it was 130 grams! I ordered one thinking he was smoking something but it was correct, it has a tiny .490 diam wrist pin and the piston has a smallish skirt and is a real nice piece. Total weight with pin clips and rings was 184.5. That's what I will go with.
So did a little more digging with this new rod combo vs the old rod and piston. I first calculated the weight of the small end of the rod and the complete piston for the stock predator. when I weighed the small end it came out to 34G and the big end 94G. If I weigh the whole rod it was 137G so there was a 9 gram difference so I calculated a multiplier by dividing 137 by 128 and came up with 1.07. So final rod weight for my calculation is 100.5 for big end and 36.3 for the small end. So now I take 100.5 Plus a percentage of 36.3 + 200 or 236.3 for reciprocating. If I use a 50% balance factor it would be 100.5 + (.5*236.3) or 118G total balance weight would be 218.5. When I put the crank with bearings (seals removed to reduce friction) on v-blocks it took around 197 grams to get it not to move and stay wherever I put it. So using that weight the stock balance factor comes out to 42%. This seems low to me and would think it would be more around 50% to keep vibrations even all around. Anyway I think because the new wiesco piston is 18 grams lighter than the stock piston but the big end of the rod is (with correction added + bearings) 117G so 17G heavier than the stock rod. I'm gonna call it a wash although it seems I could add about half an ounce on each counterweight to get it around 50%.



Active Member
Wondering how many people are balancing their crankshaft/piston combinations here. I've had to pull the head on my modded motor & am thinking it would benefit from a balance. I've never done this, but am interested in how much it would help. Right now it shakes pretty badly at 3800-4000 rpm, then it smooths out at higher r's.

The basic build is 60363, Arc flywheel & rod, Bullfrog 11:1 domed hemi piston, Black Mamba cam, chromoly pushrods, VM22-133, 26# springs. The new head will be ported a bit when it gets here & I don't have weights on the rod-piston combination. Still a stock crank which has turned 8400 & still seems tight on the bottom. Cylinder is clean & everything is working well enough, except for the head.

I didn't Fit the bracket well enough on the header at the head & the combination of forces due to the slight misalignment & vibration tore a chunk out of the head this weekend. Re-did the header bracket to pick up the lower head screw, so hopefully this will be better. Here's what the old head looks like now with the bolt hole "missing" under the valve cover.

View attachment 245095

What's going on with people doing a balanced crank on these motors? The Bullfrog piston must be heavier than a stock one, but I'm trying to decide if I should tear down the whole motor & start from scratch again or just put the new head back on once it's ported & call it good.

What if I need extra weight in the crank, tungsten plugs & welding? Shave the bottom of the piston inside a bit?

Thanks in advance for helping me open the newest can of worms in my journey into speed.

Did you remove some metal as shown from the crankshaft were the connecting rod bolts on? Since you have a 212cc Hemi engine you need to remove some material from the crankshaft since the cam lobes can hit the crankshaft in that area if you do not remove some material from were the connecting rod bolts on. On the non Hemi 212cc there is more clearance between the crankshaft and the camshaft lobes so no modifications to the crankshaft is needed.
hey ole I was just ready this thread and I was wondering if you ever ran your motor yet or not and if you thought of trying to add some extra weight to the crank. for example I believe they use a tungsten material.
No have not finished it ran into some issues with the 356 dyno cam. It is now together with one of my ported 14cc big valve heads but I am working on a warhead 32 26 head and if it looks better on the flowbench I may swap heads.
Over the years I have built quite a few engines, V8's, V6's, inline 4's, 45 degree HD engines & I don't know how many singles. As was previously stated, correctly, to really get an engine balance correctly, it needs to be balance for the application. Here's a good example, OEM manufacturers would never balance a push mower, or a ZTR to have side to side movement, it would always have a balance factor so that the vibration was going front to back. That way the user would not notice it. Old BSA & Triumph motorcycles were balance so the forces would make the engine feel like it was hopping up & down. Only the British would do that, LOL. I once balanced my racing HD Sportster 1200 to a 60% factor. the factory used 50%. A slight mellow vibration at idle to 2500, smooth a slick at 6500, where I was running the engine. Most single cylinder motorcycle racing engines are balanced to 60%/70%, again, depending on steel frame, aluminum frame, etc. Most single cylinder Clone (Chinese) engines have a couple in the crankshaft, because the two counter weights are not the same mass. That causes a vibration as it goes in and out of residence. If it were me, I would start by cleaning up the crank on the lathe and the mill to get the counter weights as close as I could, and then balance at 60% over balance, using tungsten in the flywheel. There are a few good dynamic balancing shops around the country, the best, Automotive Balancing Service in LA unfortunately went out of business.
Just my thoughts