marked23

I've heard, in another RCU forum, that using a toothbrush handle in the exhaust port while torquing a nut on the crankshaft can damage an engine. The poster mentioned "a ringed engine" specifically, but I'm not sure that being ringed even matters. I can imagine all manner of breakage in the linkage between the crank and the piston by applying too much force with such a contraption. The forum was one of the heli forums... so we are talking about 30 size and larger up to 90.

1. Is a piston stop a far better tool than a toothbrush? Why? What does 'rings' have to do with it?
2. Is there an accepted level of torque that an engine manufacturer has approved for use with a piston lock, etc.
3. What else can you do to torque something onto the crankshaft?

-Mark

jsn

My Feedback: (8)

NEVER lock the piston via the exhaust port ringed or abc...unless you like to spend money and ruin engines. if you don't have a piston locking tool then you can use a tooth brush handle or similar, remove the carb insert the handle into the crank intake port SNUG down the crank nut. I NEVER finish tightening the nut against the piston locking tool... hold the clutch housing or what ever your putting on the crank and then tighten it down. I've never seen any kind of torque spec, I've alway went by feel... when I feel it's tight enough I stop, I've never had anything come off a crank so I must be close just in case your next question is why not just hold the clutch housing or what ever to begin with... well most clutch housings wont just slip on you have to somewhat press fit them on.


John

impactiq

My Feedback: (57)

What are looking to "torque" on the crankshaft? I'm assuming some type of hub/flywheel? Engines use a collar (or a key with a taper on the crank) to mount what ever is needed. Once you get it past tightening it with hand/wrench holding what ever you are putting on. The collar (or key) does a good of not slipping past that point. What I use is a small strap wrench to hold the hub/flywheel/etc... to give it the little extra that is needed. Also very handy to remove the nut when taking off the above. No pliers for this kid. Beats using a tooth brush or a piston stop.

BTW: I use a wooden dowel down the carb's mounting throat to take the nut down to the nub/flywheel.

impactiq

My Feedback: (57)

Beat me to it.

Hobbsy

My Feedback: (102)

On the rare occasion that it is necessary I use a popsicle stick in the exhaust port as a piston stop. It spreads the pressure over a wide area and has never caused any damage. If the rod cannot stand that amount of pressure it could never stand getting hammered 200 or 300 times a second at full throttle.

w8ye

My Feedback: (16)

On model airplanes, I've never, in 50 years, had the need for a piston stop to tighten or loosen a prop nut. There was always something to hold onto like a prop blade. I kept the threads on the crankshaft in good order so that the nut could be spun off with your fingers.

The real use for a piston stop is to approach the stop from two different rotational directions and the split the difference in order to locate a precise TDC. You don't put any undue pressure against the stop. Therefore, a piston stop is not complete without a degree wheel. They are usually used to measure timing parameters.

However, at the swap table, half the engines you pick up, to look at, have plier marks on the prop hub.

I have feared the need to use some method to keep the engine from rotating, so some years ago I purchased the wrenches shown in the post back up the way.

I'm certain the popsicle stick method would be satisfactory also.

Enjoy,

Jim

DarZeelon

Using a toothbrush, Etc., in the exhaust port, holds the piston only on the exhaust side.

If enough effort is used on the crank nut, the piston could cock and twist, losing its form.

A piston stop should be used, so it would hold the piston at its center, at about 60% up travel, so the con-rod will be at 90 degrees against the crank throw. It could still bend your con-rod, or break the crankshaft. Using the carburetor spigot hole is preferable.

Holding the prop/flywheel/Etc. is still the best and safest.

Hobbsy

My Feedback: (102)

There is no way that a piston with that much surface area to distribute the force over is going to cock sideways or deform. The engines aren't made out of putty.

DarZeelon

David,

...Not putty, but metal.

If an ordinary Sullivan starter is strong enough to break the crankshaft and to bend the con-rod, in a hydro-locked HB.40PDP engine, you can believe an aluminium piston can be deformed, by stopping it on one side.

It is enough for the toothbrush to make a small dent on the crown's corner, for the piston to be considered junk.

If while tightening the nut, you just prevent what you are trying to attach to the crankshaft from turning, you will not be risking any of the engine's internals.

In a tapered bore engine, the piston is more loose to cock, than in the lapped engines of 30 years ago.

Holding the top on the exhaust side only, could bend the thin piston skirt on the intake side. This would in turn, cause the skirt to bulge out on the front and the rear, "attaching" it to the sleeve...

TLH101

My Feedback: (90)

Why not just fill the cylinder with string through the glow plug hole. Best piston stop I have ever used to hold a crankshaft. Won't work for timing work, but will do fine for removing stubborn propnuts, and will not scar or damage any parts.

impactiq

My Feedback: (57)

In some cases of RC nitro engines. Some form of locking nut is used to hold on what is ever needed to the crankshaft. I am also into 1/8 scale on road race cars that have engines that hit the 35-40,000 RPMs, and they are constantly going back and forth between 10,000 (or so) to 35-40,000 RPMs with the clutch still engaged (between 30mph to 95+ mph then back down to 30 and do it in a snap of a fingure) so you can imagine the forces that are at play here. I have seen guys use the tooth brush method and crack the skirt of the piston due to the force that is needed to take these nuts off/put on. I have also use the piston stop method and have had it cause a weak spot in the piston that gave away. So I guess the bottom line here is, It is you engine/money. Do what ever seems fit to you to do what ever is necessary.

Boomstriker

I think someone got the procedure scewed-up along the way.
The method to hold the crank with a toothbrush handle is to jam the crankshaft crankpin or counterweight through the back plate, not on top of the piston!
NEVER stick something in the exhaust port or the glowplug hole to tighten the nut, you will destroy the rod or the piston!
The carb hole will work, but exerts much more force on the housing because the diameter is much smaller there.
On an airplane version with a prop to hold on to, you should never need to do this. Only with a heli or car engine with the flywheel that is screwed onto the crank and the nut is used as a jam does this method even apply.

Kirk

ZAGNUT

My Feedback: (1)

the rope trick mentioned above is the safest method to use if you can't grip a hub or flywheel. very handy for installing threaded drive hubs on the bigger gas engines. it's always best to bring the piston up until it just closes the exhaust port and then feed the rope in through the plug hole.

the reason the toothbrush method is especially bad for a ringed engine is because of the fragile upper ring land...very easy to break.
poking things into the intake port is also a bad idea, many engines have rather weak castings that are very easy to crack.


dave

jsn

My Feedback: (8)

I've seen more car and boat engines RUINED with the exhaust port method than I care to remember... sure it can be done but one slip or much pressure and you will screw up the skirt, piston rim or conrod. I've also seen piston locking tool's driven right through the top of the piston, I only use them when things go on easy... if I have a stubborn seating flywheel I pull the carb and lock the crank... it's more time consuming BUT less risky. what ever method you choose just make sure your final tightening or removal is done holding what ever your attaching to the crank... it's much less stressful on the engine parts.


John

impactiq

My Feedback: (57)

This is true if you use this method to crank the jamb nut all the way. The best way I found is use a wooden dowel down the intake port to thread/snug the nut up to the flywheel. The dowel takes the pressure/absorbs it so it will not crack the housing or damage the crank. Then grip the flywheel, tighten to the desired "torque" needed.