ZdZ 40 a bit tight at TDC
 

Have a ZDZ 40 that clicks when right at TDC, you can feel it on the prop when it goes through the top of the piston stroke, it kind of clicks. The engine has had some running but no great amount of carbon buildup so I don't think it is the piston hitting carbon on the head. Wondering if this is something that is a known thing about these engines.

RE: ZdZ 40 a bit tight at TDC
 

It's normal. That's what happens when the rod hits top of stroke, the piston rocks up and down a smidge with the required play in wrist pin bearing, and the big end of the rod does the same on the crank. If it didn't do that it would be too tight to turn over at all. It's called a ganging of tolerances.

Go fly and have some fun. No worries.

RE: ZdZ 40 a bit tight at TDC
 

The engine is noticably tight as it goes over TDC, but after taking it apart there is nothing obvious.

RE: ZdZ 40 a bit tight at TDC
 

Is that with the plug out? If not, ZDZ's have a higher compression ratio than other engines.

RE: ZdZ 40 a bit tight at TDC
 

Yes it is with the plug out, that is what I am wondering about.

RE: ZdZ 40 a bit tight at TDC
 

They all do it to some degree. When the crank goes over center, there is a little slop in the rod bearings that makes a click. The slop is why you have to wiggle the shaft back and forth to find TDC when setting the timing. You have to find the center of the slop. When running, the clearance is taken up by various tensioning mechanisms(Compression/firing/prop drag, etc.).

RE: ZdZ 40 a bit tight at TDC
 

It is not that hard to remove the cyl. Take the piston ring off the piston....clean all carbon out. See how tight the piston fits in top of bore without the ring. I had a DLE30 side carb that was tight enough to make a fair amout of compression...without the ring. It is fine now...I placed a Frank Bowman ring in it and it runs great. The original ring was too hard and wasnot breaking in fast enough...it was super hard ring...not a good thing. Anyway take it apart and be care full not to break the ring...unless you have another. Capt,n

RE: ZdZ 40 a bit tight at TDC
 

yes the engine is apart, the only thing I see is there is a very small ridge n the cylinder bore where the ring stops at the top of it's travel, you can just catch an exacto blade point on the ridge if you slide the blade up the bore. The tightness is not there if th e piston is temporarily removed then everything feels normal, put the ring back on and it is is just slightly TIGHT at TDC.


The engine has not had a lot of running, just broken in I would say, piston and bore look fine and so does the ring.

It is somewhat of a mystery as to what is causing the tightness.

RE: ZdZ 40 a bit tight at TDC
 

Now that is is a part any thoughts as to what might be the problem?

RE: ZdZ 40 a bit tight at TDC
 

There should be no ridge there....possibly come up with a way to remove it. Got to think on that one .Is that ridge all the way around the cylinder? If the head was removable from the cylinder...you may be able to use a ridge reamer. This would need to be done with extreme care. Capt,n

RE: ZdZ 40 a bit tight at TDC
 

The ridge is all the way around but seems more pronounced on the thrust side, I was surprised to see it there, I don't know what material ZDZ uses for cylinder wall material but there is definitely a bit of a ridge, I would estimate that it is less than .003 inches on a side..

RE: ZdZ 40 a bit tight at TDC
 

you might try a tad thicker base gasket....just to see how it runs then. I sure would place a Frank Bowman ring in it as long as you got it apart. Capt,n

RE: ZdZ 40 a bit tight at TDC
 

Thicker gasket is a good idea, would like to know what material the cylinder liner is made of and if it is normal to see this much wear in such a short time.

RE: ZdZ 40 a bit tight at TDC
 

It may be possible that your engine,s sleeve did not get coated good with whatever they use to harden the surface. See if you can have some person do a Rockwell hardness test. Maybe you will get a new replacement...free...except for shipping???

RE: ZdZ 40 a bit tight at TDC
 

That "ridge" is not a ridge at all. It's where the cylinder is cut just a little bit wider than the rest of the bore and the ring never, ever gets there unless you remove it and shove it up into the expanded area.

As I said before, the condition is normal. However, if you want to worry without cause please feel free.

RE: ZdZ 40 a bit tight at TDC
 

Even if the ridge was from wear both the ring and Cyl would have worn in together and you wouldn't be able to feel any catching on a ridge unless a new ring was installed with a sharp edge. That is why the ridge was removed back in the day of ovehauls to prevent the new rings from being busted the first time the engine was turned over.
TOM is saying it's and undercut, are you saying it is not? In other words the bore is worn larger below the ridge?
You can add a small piece of .010 solder on the top of the piston and turn the engine over to check piston clearance, or modelers clay will work also.
Like has been already said add an extra base gasket or some sort of thin spacer and see if the issue is still there if not then I would look into maybe some warranty.

RE: ZdZ 40 a bit tight at TDC
 

Yes the bore is worn larger below the ridge. The tightness goes away if the ring is temporarily removed, I estimate the ridge is about .002 to .003 inches. The tightness at the top of the stroke is quite noticeable. The piston and ring look good, no scores in the bore.