One or two-piece crankshafts for big nitro engines?

[Milner]

Hi guys,

As you can tell by my post count I'm a new here. I've done a ton of reading since I discovered the forum a month or so ago. Let me say that I think this forum ROCKS. The depth of knowledge and experience is second to none that I've found on any message board, regardless of topic. Thanks to all who contribute.

So now to my question...I've done some searching using keywords like "crankshaft, crank and engine" (the three letter search limit prevents searching for engine names like MAC and CMB) and while I found lots of information I didn't really find anything that suggested why one might choose an engine with a two-piece crankshaft over an equivalent with a singe-piece crank. By two-piece crank I mean like the design used in the CMB 91 and 101 "RS". It appears that the CMB 91 "H-Racing" has similar specs, but with a single piece crank. I do realize the induction systems are different also. Do the structural benefits of a two-piece crank outweigh what seems like added assembly and maintenance hassle? What do you guys consider to be the "state of the art"? I ask this just because I'm curious about engine design in general and have limited experience with these larger and very high output engines. Most of my experience has been with car and sport airplane engines which aren't generally leaned on nearly as hard.

If this has been discussed somewhere that I missed please feel free to point me in the right direction. Thanks.

Brent

 

[moparbarn]

if the 2 piece crank is welded, i'd be ok with it. if it's pressed together, i'd steer clear...........

 

[Ray Sametz]

Welcome aboard Brent, I only run one piece cranks, I hear the maintenance on the two piece is a bear. Some of the engine gurus will respond for sure. B) B) B)

 

[RaceMechaniX]

I have both styles and find that so far the CMB RS cranks are pretty stout with the exception of the 35RS gas motor, but thats another ball of wax. I don't weld any of mine and so far the 91 and 101 are still true as the day they began running. I will say that I have not had a bad dunking though which more then anything probably causes the mutlipiece cranks to twist.

TG

 

[Buckshot]

All my .90 and 1.0 motors are evolution, double flywheel style cranks. I have twisted only one about the crank pin and completely shattered a crank pin in another in the 7-8 years that I have been running them. There really isn't any difference maintenance wise between the two if they are in good working order. To fix a double flywheel crank that has been twisted is pretty tough but can be done.

Performance wise I couldn’t tell you which one is better. I have seen only a hand full of the newer .91 Hr motors run and can’t say I was totally impressed. That doesn’t mean that they are not good.

-Buck-

 

[John Knight]

I have both style engines and both will run quite well. The disc engines (CMB RS styles) are more compact from front to rear and are slightly heavier. The CMB EVO (mine are 90's) cranks are two piece and press fit. The bearings generally last a long time due to the excellent balance of the engines. They are more of a challenge to take apart but with very good maintenance it is not frequent to have to do. Never broken a crank or rod.

The hydro style single crank, (MAC, CMB HR, Picco EXR late models, etc) are much easier to take apart and service. I have needed to replace the bearings about 2X more frequent with the single crank vice the EVO split crank engines. I attribute that to the design difference between the two types. My maintenance is identical for all engines so that is not a factor.

With that being said, both are excellent type engines. The key to it is an excellent maintenance and cleaning program to ensure long life. If you constantly lean out the engine trying to squeeze that last bit of speed, no engine will last long. Run it slightly rich and put in a good maintenance program and they will last. I have one CMB EVO1 that was new in 1995 and it still runs excellent, with the original piston and sleeve. A new one is finally going in soon.

 

[Milner]

Thanks for all the replies. I guess I was thinking that the bottom end of the rod might still need fairly short service intervals like most highly stressed bushed setups. It sounds like the steel rods with needles tend to last a while.

Being new I wasn't sure if one of the designs was becoming obsolete for these larger nitro engines.

John, your comment about the bearing life is interesting. I hadn't considered that the main bearings would be better off with the 2-piece design.

If you do need to replace a rod and needles, do people typically disassemble the crank or is it more common (or even possible or economical) to buy a complete pre-assembled crank/rod setup? I know with ignition 2-strokes (dirt bikes) that complete cranks assemblies are often replaced rather than disassembling.

It sounds like the general consensus is that the two-piece setup is generally preferred on these larger engines and that service isn't needed very often assuming you care for the engine properly.

Any other considerations why you'd choose one over the other are appreciated. Induction type?

Brent

 

[lohring]

In higher powered and larger engines, crankshafts with bearings on both sides of the crank pin (two piece construction in our engines) are the only durable option. Motorcycle mechanics think nothing about pressing apart cranks for maintenance. It usually is a better option to buy a complete crankshaft and rod assembly for Zenoahs since the cost is modest.

When overhung design crankshafts were used in gasoline engine model boat applications (Homelite engines) the crankshafts fail as the engines were modified for higher than stock power. In contrast, the basic Zenoah crankshaft and rod started life in a 1 1/2 hp engine turning less than 5000 rpm. It now is used in engines generating 8 hp at 19,000 rpm. Only with prolonged operation at over 20,000 rpm does the rod bearing begin to fail. The CMB 35 and Zenoahs still suffer from crankshaft flexing at high loads and high rpm. We fixed the CMB with an additional bearing on the drive side. Others have improved the Zenoah with three and four bearing modifications. Quickdraw has sometimes run four bearings. Only in smaller engines is it possible to build crankshafts rigid enough. I think 15cc engines are about the largest size this is possible in for really high power applications, even though much larger model aircraft engines have overhung cranks.

Lohring Miller

 

[ids987]

Even Picco changed to split crank for the late versions of the P90

http://pagesperso-orange.fr/xavier.cardina..._90_2003US.html

http://www.teampicco.com/Motori/p90-xpmarine.html