crank bob weight

[Robie6]

This is probably an off the wall question but here goes anywho. I'm working on a spare Torq 21 mill thats been sitting around & use it as a practice engine to better my skills in porting & balancing. This is my 3rd engine & the other 2 run great with slight mods I've done including case-sleeve matching, induction port timing & carb relieving. So what I've done so far to the 21 is hone the case to except a XTM 24.7 sleeve & piston using the 21 crank & rod & pin & match the case-sleeve ports. In trying to balance the crank I weighed the piston, pin & clips on a gram scale (5 grams) the small rod end (1 gram) the big rod end (2 grams added the piston assembly to the rod small end to get 6 grams. Using a 50% factor & adding this (3 grams) to the rod big end weight of 2 grams I end up with a bob weight of 5 grams. I made up a balancer from a old prop balancer with magnets & pins for each end of the crank. Set up- one end of the crank just touches one magnet & the other end "floats" off the other magnet about 1/16". With the 5 gram bob weight on the crank pin it shows the crank real heavy on the pin side opposite the counter weight to where it looks like I'll have to remove way to much metal around the pin area to get a balance. Could I put a bead of weld on the counter weight & grind this to compisate taking weight off the pin side? There's room between the counter balance end & the case for this. The 24.7 crank counter weight is bigger than the 21 & comes real close to the case. Thanks in adv. for any help & direction, Robie6

 

[scottm]

Wow, you are a dedicated hot rodder. I don't know the theory of engine counterweight balance, but I know that it doesn't take much to throw something off. I balance plane props with just a touch of spray paint. A 1:1 car driveshaft is thrown out by paint or undercoat overspray. So a crank spinning at 30,000 rpm will be very sensitive. You might try EBmods or some other porting shop. Steve Pond is an engine expert, Tech editor at RCCA magazine, and admin at radiocontrolzone. Start a thread and ask him. I don't think you will get torched, they are pretty cool there.

http://www.radiocontrolzone.com/forums/forumdisplay.php?s=&forumid=218

 

[Robie6]

t/y scottm, its fairly easy enough to understand. I worked with a guy who races a top fuel Harley & he does balance jobs for customers & taught me a few things, not that I'm good enough to start balancing mills for r/c people. I have most of the tools & shop to get started, just bought a gram scale the other day. Reason I shoved a 24.7 sleeve in a Tork 21 is I have 4 engines now. The .21, a 24.7, a .26, & a .27 & they all use the same 3.5 block, bearings, ports in the block are close enough in all to just do a little matching. If XTM can plug a 24.7 sleeve in a 3.5 block why can't I? With dial calibers, degree wheels, honing machine, bore guages, & all the rest of the stuff I've have in my box I figure I have a pretty good start. Even If I blow a test engine (my own) I'm still learning-right? Ya haven't seen nothin till ya see a 400hp fuel injected nitro Harley blow a head off the line-lol. I did email EBmods a couple weeks ago about some things I'm trying to do but never got a reply, thats kewl

 

[wdavidhicks]

I think you might be working your way into an excellent trade, Robie. If you have friends who are into nitro they might give you old engines to hone your skills on. When you turn out the successes they'll keep giving you engines. Not long after that you might hang out the shingle and get out the cash register.

 

[ironman]

Some notes on crank balance:

Hi.I copy this from the net.Hope this might help you.I'll post another tips on sleeve porting later.

If you want to get fussy, and allow for the port being moved, balance on a "wheel" type balancer. This can even help on a stock type crank. (see some of the Russian engines, .15 size for examples). The amount of counterbalance needs to be in the 50~65% range of the weight of the piston,wrist-pin,(ring, if used) and 1/2 the weight of the rod. To test this, measure the weight of the reciprocating parts on a good gram scale, and make up bob-weights in this amount to add to the crankpin, before you put it on the balancer.The crankpin should "stop" at the 12 O'clock position. On a single cylinder engine: If you add weight to the counterbalance, or lighten the reciprocating weight. you DECREASE the PRIMARY shaking imbalance (vertical= Cyl. bore axis), and INCREASE the SECONDARY imbalance (crosswise= across the lugs).
For this reason, IT'S IMPOSSIBLE to balance a single Cylinder engine, without a secondary Balance shaft, as used in some auto engines. Adding weights (heavy slugs) to the counterbalance is done to change the resonant frequency at which you get a "Harmonic" or strong vibration level. You want this at a lower than full rpm number. Most of the ST X-40's used in Formula-1 Pylon and Fast Rat had cranks that needed this changed to resonate in the high-teens (RPM). Sometimes you could see the engine "shake" in this range, on the test stand. I never put a strobe light on one to see what one would look like while running. If the engine is balanced to "resonate" at full RPM, IT USUALLY COMES APART

 

[Robie6]

Thanks wdavidhicks for the support & thank you ironman for the information. Your right- a single cylinder is imposible to balance however engine makers to make things simple use a average figure because they dont know who using the engine, where its used (climate) or the conditions its used in. A crank balance of 25% is the norm in most r/c 2-strokes. If you balance a crank to 50% thats an improvement. Some really good modification people can get up to 75% but were talking computer programs & the one program I found was about $300.00. Maybe once I learn more, I don't want to jump ahead of myself. 50-60% is a good figure, more forgiving for me now. I found some good information on a r/c racing boat site while surfing around. These guys do alot of there own modifying. They have 2-strokes like our 3.5's that put out 5 1/2hp! I dont know how but the engines sell for about $700.00. can't be used in a car cause the intake is in the backplate & the head is watercooled but the head can be changed. I'm not sure about the front of the crank, anyway it be interesting to see a 6hp 2-stroke hydroplane engine in a m/t or buggy. I doubt if you'll get 25' before the trany blows a spur gear or worse. Interesting that these engines only rev to 30,000 rpm. Anyway I fitted the 24.7 sleeve & piston in the .21 last night. The pinch on the piston at TDC is good, I opened the closing side of the induction port to 65 degrees & got the crank within 50% balance (using a "floater" balancer) & with a .010 shim I have .026" clearance from button to top of piston (squinch) Going to use 20% fuel for now. Think I should wear a bullet proof vest & helmet for the 1st run up??-lol :tank: