051 Head Scratcher

Wiztom Fri Sep 25, 2015 7:36 pm

Howdy,

I have 3 .051 Tee Dee and One .049 Tee Dee engines. They have all been run.
I have one .051 that quit running. When I took it apart the piston rod was bent. (my bad using low torque starter when flooded).
I have replaced 1. piston and rod. 2. Head, 3. Crankshaft. (in that order). None of them have solved my problem.
When turning the prop there is a "notchy" (tight) feel at TDC, and I can actually turn the prop back and forth a millimeter or so without moving the piston.
None of my other engines have this. They turn smoothly all the way around and there is no notchy feel or slack in the prop at TDC.
Hopefully I am describing this well.
Anyone have any idea?

Thanks

Ken Cook Fri Sep 25, 2015 7:43 pm

How did you install the cylinder? Did you insert the wrench through the exhaust ports to tighten? It certainly just sounds like the piston is just tight at TDC and the rod is snapping over the high point of the crank which is why the piston isn't moving. A little piston lapping might be necessary. I do this with the piston upside down in the cylinder.

happydad Fri Sep 25, 2015 10:13 pm

Ken Cook wrote:How did you install the cylinder? Did you insert the wrench through the exhaust ports to tighten? It certainly just sounds like the piston is just tight at TDC and the rod is snapping over the high point of the crank which is why the piston isn't moving. A little piston lapping might be necessary. I do this with the piston upside down in the cylinder.

Right on Ken. +1 for that. Wiztom: Look inside the cylinder for burrs, top to bottom, or bottom to top.

happydad RC Plane

Oldenginerod Fri Sep 25, 2015 11:32 pm

Try adding an extra head gasket one at a time. Maybe the piston is a little "taller" than the original and it's contacting the head.

Rod.

Ken Cook Sat Sep 26, 2015 12:24 am

In addition to what Rod was suggesting and a good point indeed, check the sub induction under the piston skirt. It really should be no greater than .012". If so, placing shims under the cylinder would maximize performance and kill two birds with one stone. Obviously if these are not available to you, the additional head gaskets are the simplest suggestion. Kustom Kraftmanship offered these in a bag of about 6 which had a variety of shims. It was common in the 80's that Cox would provide 3 head gaskets and you removed one after a few minutes of run in. My experience has shown that head gasket thickness is all over the map. Without taking my new ones apart I couldn't tell you what thickness came with those engines. I found differences as great as .004" with head gaskets. Some of the newer available gaskets are as great as .008". For this reason it may require more or less. Truthfully I think Rod has nailed your problem down. Don't forget to check your collet nut that holds the carb body on. These can loosen backing out onto the prop drive shim and pinch against the drive washer which can shut a engine off immediately. The reason I mentioned the collet tightness is due to your use of a electric starter. Ken

Oldenginerod Sat Sep 26, 2015 1:00 am

A while back I bought two new Surestarts and one of them (die-cast case) was doing exactly what "Wiztom" described. Being Surestarts I generally will use only one head gasket. In this instance I removed both cylinders and swapped them one engine to the other. The TDC "bump" had gone from the suspect engine and the suspect cylinder seemed to work fine on the other engine, so there clearly must have been some machining inconsistencies.

happydad Sat Sep 26, 2015 1:13 am

Ken Cook wrote:In addition to what Rod was suggesting and a good point indeed, check the sub induction under the piston skirt. It really should be no greater than .012". If so, placing shims under the cylinder would maximize performance and kill two birds with one stone. Obviously if these are not available to you, the additional head gaskets are the simplest suggestion. Kustom Kraftmanship offered these in a bag of about 6 which had a variety of shims. It was common in the 80's that Cox would provide 3 head gaskets and you removed one after a few minutes of run in. My experience has shown that head gasket thickness is all over the map. Without taking my new ones apart I couldn't tell you what thickness came with those engines. I found differences as great as .004" with head gaskets. Some of the newer available gaskets are as great as .008". For this reason it may require more or less. Truthfully I think Rod has nailed your problem down. Don't forget to check your collet nut that holds the carb body on. These can loosen backing out onto the prop drive shim and pinch against the drive washer which can shut a engine off immediately. The reason I mentioned the collet tightness is due to your use of a electric starter. Ken

I have the cylinder shims I purchased over the last 2 years for a very low price. If you need help, please feel free to PM me, private message, and I can help you out.

Matt at ESP-... Has the shims for $2.95 + $.75 shipping. I didn't have time to check cox engines.ca, but Bernie most likely has the cylinder shims as well.

Happydad. RC Plane

balogh Sat Sep 26, 2015 1:21 am

You can check the TDC bump without the glow head on. If it stays you have burrs in the cylinder. If not then your piston is tall.

Check also the crank radial play. Some misused TD-s develop this if run with unbalanced propeller for long. The result is sloppy crank fit= impaired fuel drawing capability and difficult starting.

fit90 Sat Sep 26, 2015 1:00 pm

The "notchy feeling" at TDC could also be because the conrod to piston ball socket fit is loose. If it is loose it makes starting far more difficult than usual. Hopefully it is something simple and inexpensive. Good luck

fredvon4 Sat Sep 26, 2015 4:37 pm

quote

"When turning the prop there is a "notchy" (tight) feel at TDC, and I can actually turn the prop back and forth a millimeter or so without moving the piston. "

Could simply be a nice pinch on top dead center ---not necessarily a bad thing--- and the rocking is simply the rod socket play..

Some rod/piston bearing clearance is usually noticeable and I don't bother re-setting the ball end until it is fairly sloppy like about a 33 degree prop arc from loose up to loose down.

My re-set sets still let the prop move about 10 degrees up to down slop and I think I always re-set too tight

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