After pushing my luck with the red bike engine, I've decided to build a better motor. Some of the parts will be from the first one, other parts will be new. For now I'm planning on using the same block, Bullfrog piston, ARC rod & flywheel, head (the new one I just installed) & the Black Mamba cam. I've sent for some stainless steel valves from ARC, with new springs, keepers & retainers.
I don't trust the stems on the original valves, as they're pretty thin up top & I've pushed them too hard at this point to trust them. If I break a stem at the top, even at a low speed, the valve will drop into the piston & the motor would be completely destroyed in one turn of the crank. The new valves should take care of this potential problem before it happens.
I had a 60363 motor from a sale last year, so today I stripped it down. One thing I'm planning on doing is replacing the crankshaft in the new build. I had "fixed" the first crank by sanding it down by hand, as it was about .005" out of round. It was a real PITA & took a lot of time & effort to make it right. This worked pretty well, but I'm not going to take any chances with this build. I checked the crank on the new motor & found it's pretty close to round, only out by about .0005" as close as I can measure, so it's going to be a keeper. It will be much easier to just install new bearings & put it back together.
I've been looking into balancing this motor, as the original one had a bad case of the shakes which would go away at higher rpms, but it was pretty uncomfortable when it got into the speed where it vibrated badly. You either wanted to run faster or slower, but not in that rpm range.
I'll be checking the Bullfrog piston for weight as a part of the balance, but I ran the numbers with the stock piston, crank & rod, just to see where this motor comes in from the factory. The piston, rings, wrist pin & clips weighs in at 201.8 grams. The small end of the rod is 32.6 grams. The amount of mass needed to counter-balance the crank & rod for stability is only 59.2 grams. Not a lot of weight really, compared to the piston.
The "normal" balance factor for a single cylinder engine is about 50%. This can vary up or down by a few percent due to cylinder angle & a bunch of other stuff, but the 50% number is typically considered to be what works well. When I ran the numbers for the factory piston, rod & crank, it came out to only a 39% balance factor, so it's not too good from the start. They build this motor way out of balance at the factory. Can't wait to see what it looks like with the Bullfrog piston set up to run. I'm thinking that the Bullfrog piston is heavier than the stock one (not sure about that yet), which would explain the bad vibration problem I had. I'll get back on this as I find out more & start taking down the old motor.
Here's the info on the ARC/Bullfrog (Arias) piston & rod combo.
BF Piston mass 253g & ARC rod 149.5g Total 401g
The HF piston 201.8g & rod 142.3g Total 344.1g
You can see there's difference of about 56 grams & the ARC/Bullfrog combination is much heavier than the original factory piston/rod combination. I'm guessing this is where I picked up the extra vibration on the built motor. That's a lot of mass moving around at 8000 rpm, but 4000 is where the shakes are pretty bad, glad it gets smoother as it spins faster, or I wouldn't be able to hold onto it at speed. It settles down nicely once it climbs out of the shake range.
Forgot to mention, the end play on the crank has opened up from about 0.010" to 0.014" so it's doing OK. Would expect it to be looser over time & hard running, but I'm hoping the new crank ends up being about the same with the same gasket thickness. I can adjust if I need to with a different gasket, but the 0.010" I was shooting for seems to have held up well enough after the high speed runs.
Here's what the piston looks like after running for a while. No problems with valves hitting or floating against the recesses. There's a slight ding/gouge in the spark plug dimple, but it's nothing a small amount of scooping with the die grinder & a burr won't take care of. Here's the spot when the head was first pulled.
Here it is prior to smoothing it out a little but after a little bit of cleaning & buffing. It really looks to be fine, no splits & a minimum of damage to the piston, considering what I did with the 8720 rpm run last time there's no real damage.
I'm pleased with what I found when I pulled apart the bottom end. The crank journal still looks pretty good & the bearings are ok, but a bit worn & uneven in places. They look ok, but not as good as they were before the motor was run hard. I'm still going to replace the crank & put in new rod bearings, but this motor had a lot of life left in it when I tore it down for inspection. The numbers on the main journal vary between 1.187" & 1.189" so I was pushing my luck with the crank being out of round & turning it as fast as I did.
I'll order a new set of bearings for the new crank, replace the valves with the SS ones when the arrive & then button it up with new gaskets. All in all it's still a good motor & should hold together well enough if I stay at about 8000 rpms & don't over rev it next time I have it out. I'm going to bump the timing a few degrees for the altitude here, since I have the flywheel out already, but otherwise, it's going to be about the same thing once it's back in the bike again. I'll change the gearing a bit also, to make it harder to pull so many rpms but it should still top out at 65 mph & above.
Had a little more time in the shop this afternoon, so I worked on the piston to clean it up & work out the ding in the plug recess. Basically, I just chucked a burr into the drill press, ran it fairly slowly & carved out the bad spot freehand. I cleaned it up & then put it on the buffer to shine it up a little. It's about ready to go back in the block again, but I'm waiting for the new bearings from ARC to arrive. It won't get set until I have a chance to plasti-gauge the bearings for clearance, but it should come out to about .003" & be good once it's checked. Here's the piston cleaned up a bit & ready.
It's not 100% polished, but it's better than the last time it ran. I'm still pondering the whole balance thing. I'd need to add more weight to the crank, but it's not going to happen on this build. My weighing procedures aren't very good due to friction in the bearing areas & since it ran well enough as it was, I'm not going to open that new can of worms. Perhaps if I had access to a machine shop it would be different, but with basic wood working skills & tools, it's not something I'm going to try at this point. On a different build (460cc?) I may look into it again. Will post again once the assembly starts.
I got another head & decided to start over since I have time while I wait for the bearings to arrive from ARC for the new crank. The new SS valves arrived along with the new head, so I decided to try a little porting prior to assembling the head. Have to say a Makita die grinder is really the right tool for the job. I had done the replacement head with it & this time it was much easier than the first time. Only took a half hour to smooth things out & prep the head at the intake & exhaust, so I'm improving as I get more experience.
You can see I still need to refine my technique (gouges & don't have the right tools for polishing inside the exhaust better), but the basic port should flow a lot better at higher rpms and it's much better than the factory lumps, bumps & restricted bowl area. At least it's cleaner now than it was. I put the new valves, keepers, retainers & springs back in place. Put the rocker arms back & stuck the lash caps on with assembly lube. Once the bearings get here I can check the cam against the new crank, re-mount the billet flywheel (have to lap it for the fit) & start assembly.
I'm getting rid of the stock gas tank & have ordered a new 3x8 spun aluminum one from Sandstorm. I'm thinking that having it sit higher over the carb will help with fuel flow a little bit. It only holds about a quart, but it's not a bike I will be touring on or riding around the dirt, so it should be plenty of fuel for a few runs at a time. It will be a lot easier to add fuel with this mount. The under-seat filler is a PITA to work with compared to having it out in the open.
Follow-up on the new bottom end. I didn't do anything with the balance, too much technology for a wood worker, so it's back together with the extra 50 grams of piston weight. It worked once, so it should work again.
I used the new ARC bearings, but this time the HF crank is tight (never right with these things). It comes out at about 0.0018" on the main journal, but ARC recommends it to be at 0.0025-0.0035. I looked it up online & have found that while it's pretty tight, it should run OK with 0w-20 weight oil, so it's being switched over to the Mobile-1 once I drain it of the 5w-30 that's in it for the run-in period. I'm keeping the r's down a bit & waiting to do a hard pull when the new oil is in place & it's been run at lower rpms & heated up a bit.
Had a problem with the float when I tried starting it. Fuel was leaking out of the overflow, so I pulled the carb & found this in the needle valve. Not sure how it got there, but it didn't run with this in the orifice.
I added the new fuel tank to the frame & may still run a pump to pressurize the system, but the new fuel line is an upgrade from the old black rubber hose it had before. The tank only holds a quart, but should be good enough for a few runs. Everything seems to be working again as it should. New SS valves are better & shouldn't snap off at the top of the stem. Still running (new) 26# springs & the Black Mamba cam. It really wants to pull with the new head & everything working well.
I broke a spring on the new motor. Was running a test pass & at 6800 rpms it broke the exhaust valve spring. Fortunately, it didn't suck the valve into the piston, as the keeper did its job & stayed in place on the valve stem. No piston to valve damage, no other problem, so I dodged the bullet again this time.
I spoke with Jody at ARC Racing & went over what I did on the build (the new stainless steel valve kit) & told him what I had put together. After looking into the replaced valve spring, checking for coil bind, measuring as best I could for the 0.85" spring height, it's back running again. There's over 0.06" of gap between coils, minimum according to ARC is 0.040" so it should be fine now.
I've taken it up to 7,300 rpm so far without any trouble. Need to to a full speed run up to 8000 again to make sure it's fixed properly & running, but it seems to be ok for now. Will post after the next full pass.