TD backplate shim

Thanks heaps Levent. another simple and available solution. Its getting easier all the time now.

Thank you Yabby for validating the idea...the 049 cylinder shim was also one option I thought of, but because this is an unobtainium nowadays, I dropped the idea.

I am not sure about US thread sizes, but maybe a standard washer the size of backplate thread could also be an option, readily available from a screw shop?. In that case probably only one instead of two thrust washers under the drop drive plate would suffice?

Forgot to mention that, gasket need to be screwed on threads, simply turn the gasket clockwise.

no worries! that means its a good fit

One grey gasket thickness is 0.2 mm, perhaps two gaskets are enough.

balogh wrote:Thank you Yabby for validating the idea...the 049 cylinder shim was also one ophion I thought of, but because this is an unobtainium nowadays, I dropped the idea.

I am not sure about US thread sizes, but maybe a standard washer the size of backplate thread could also be an option, readily available from a screw shop?. In that case probably only one instead of two thrust washers under the drop drive plate would suffice?

There are several workeable options now using easily obtained parts. I will take a backplate and go rummage arounf some old hardware stores, but Im happy to get some shimms made by a local engineering company, and I will order the gasket Levent found fitted and see how much clearance I gain from that. The gasket might be the simplest solution yet. We will find out. The good thing is no more grinding metal shimms and gaskets, much better and we allready have several options worked up in a day or two of looking into it.

[quote="Levent Suberk"]One grey gasket thickness is 0.2 mm, perhaps two gaskets are enough.

Perfect!! Thanks for mesuring Levent ! Looks like I just need to order some gaskets. Thats easy. 2 gaskets, 2 washers on front of crank, pull up prop plate nicely and will have almost .5mm clearance. Even I cant wreck that.

Hi Andras @balogh , Levent @Levent Suberk, Roddie @roddie,

To confirm the Cylinder shim ground out with a dremel worked as a backplate shim on the TD 049, I actually ran the motor with the ground out Cylinder shim fitted to the backplate today and it started and ran perfectly. Other solutions such as gaskets and or custom made shims obviously will work also, but now the ground out Cylinder shim is a proven solution.

Hi Yabby, I am glad you have found the simplest solution that did not necessitate any surgery on the backplate.

thanks Andras! yes it is the simplest and nest solution by far. Quite happy I can now easily mod my TDs to my liking and return them to standard if needed for some reaon.

I've read bits and pieces of this post and I have to say I'm a bit curious as to the need for this shim. I also recall reading that you bent the crankpin. I'm not doubting you in anyway but in my lifetime, I've never seen this or even thought of that being possible. The crank and rod are both hardened. Generally, parts like this don't bend, they break. If these parts are bending, they were not properly heat treated. I used the TD in 1/2A combat for quite some time and these engines were all out and hit the ground on a regular basis. I never witnessed a crank break. What I have seen is the crank crack across the square cutout within the crank and it was still running. Not well, but still running. Rich Lopez had a article which essentially came from Joe Klauss in regards to insuring that the crank was hardened correctly. This was heating the crank in a oven for at least 45 min at 450 deg. To insure proper hardness, one was to look for a bluish purplish tinge on the crankweb and then let it air cool.

I personally never done this because I never had the need to. However, I've seen the crankpin/rod rub onto the backplate and I would just lightly sand and polish this out. If this was a problem that would occur with frequency. I would probably opt for a shim but again, there's generally enough room in there to accommodate.

Shimming the backplate off of the crank seems counterproductive to me. It does somewhat rely on that face of the plate to be there to insure it stays put. The crankpin in theory is supposed to run true radially but that isn't always the situation. As the crank wears in the case and under certain loads, the crank is flopping around so to speak. While you may think you have some known clearance in one spot, it certainly could be greater in another.

Hey Ken, I dont disagree with anything you are saying and I certainly find it unusual what happened to the three engines. If it wasnt for replacing the cranks making the missfire / want to backfire on start go away I would think it was something else. The back plates were heavily marked/dented not just a scuff. The other thing that is odd is that huge numbers of these engines have been sold and used as you say in combat etc. and crashed heavily and no one has ever brought it up before. If it was happening you would expect it to be far more common and well known. I honestly dont understand properly myself. All three of the engines after being crashed I managed to get running by adding heaps of head gaskets to them, but then the head would come loose all the time on them. ( the prop had been replaced ) and they didnt run or start well anyway, but replacing the crank fixed the problem on each.

I am in the process over time of rebuilding three engines using the cranks I suspected the pin of bending on to see if I can reproduce the bad run they all gave.

The engines that had the problem were engines I had assembled mysellf and I suspect that may have been the problem. They are not engines that were bought as complete assembled engines. I wonder if I left the crank prop plate really loose on them. I should / do know better than to do that. but...........

probably as I build three engines using those cranks again they might run fine. I hope so. The engines were not crashed into grass, they were crashed into hard baked dolomite and clay paddoc that was like concrete in the middle of summer.

I wish I knew the answer. all of my flying is done with TD 049s and they all go great, reliable, start well, great engines, and I fly every second day generally so they get use.

Ken Cook wrote:      
              Shimming the backplate off of the crank seems counterproductive to me. It does somewhat rely on that face of the plate to be there to insure it stays put. The crankpin in theory is supposed to run true radially but that isn't always the situation. As the crank wears in the case and under certain loads, the crank is flopping around so to speak. While you may think you have some known clearance in one spot, it certainly could be greater in another.

Just re-read your post Ken. The quoted part is interesting food for thought. I had never considered the plate helping keep the big end stay put, but as you describe it, it makes sense! Hmmmm

With the semi-circle cross sectional groove machined into the crankcase just along the track of the big end, except for the TDC area, the big end is retained by the groove, in my opinion, no matter how sloppy the pin has worn..( see the photo below) and the backplate will retain it further, when the big end has left the crankcase groove near TDC...in fact when the cylinder and backplate both are removed, you cannot slip the big end off the pin unless you crank it up to where it is out of the groove already, near TDC. This is true for all COX crankcases, reedies and front rotary valve designs both.



Yabby as the TD engines evolved, at a certain point of time the web was made thicker in order to prevent the pin from snapping, experienced in competition situations. The thicker web takes the modified crankcase with a No2 punched into its left engine mounting bar (see also in the photo above) . Combining a new thick webbed shaft with an old stock production crankcase NOT having the No 2 on its mounting tab may result in reduced axial play and the shaft retracted into the crankcase excessively, that may also lead to the pin touching the backplate? Just saying because your 3 home built engines may have these unwanted shaft and case combinations? I suggest you check the web thickness and the No 2 imprint on the mounting tab..

Perhaps propeller drive plate loosened on impact.

I hardly slipped a grey gasket between web and backplate while pushing crank to back. Clearance is about 0.2 mm.

Thanks Levent. Yes, the drive plate slipping and letting the crank move on impact is my best guess. all three impacts were 90 degrees straight into the ground. To be honest Im surprised that the engine wasnt a pancake after that and all it did was to seemingly bend a crankpin. Yes, the clearance is very tight. I wonder if I might have reused prop plates on those engines. I believe reusing them they dont hold as tight.

Hi Andras @balogh , none of the CKs I own, have anything stamped on them. Nearly all of them I bought as CK assemblies and have put later production SPI cyliners with no stamp onto them. I have looked at all of my cranks and the webbing is the same on all of them. The situation you describe may be whats happening Im not sure. They are recent CK purchases so I assumed the correct pieces are in them. But they do run very close to the plate. I dont think shimming a .4mm max gap between pin and backplate will do any harm and may help stop it striking in a crash.

They do run very well though.

For the crank to push back and hit the back plate with a prop installed would mean that the prop and or washer/spinner was crushed and force transmitted through the prop screw. It would be the same process as pushing the on the crank to remove the prop drive hub. Any of the other assembly errors would have been fairly obvious due to excessive end play.

Rods rubbing the back plate is common for a variety of reasons. And, there are several designs to prevent it.

Norvel and related engines all have large clearances to the back plate. Most seem to have no trouble, but I have a couple rods that clearly liked to run 1/3 off the crank pin.

Yabby wrote:Hi Andras @balogh , none of the CKs I own, have anything stamped on them. Nearly all of them I bought as CK assemblies and have put later production SPI cyliners with no stamp onto them. I have looked at all of my cranks and the webbing is the same on all of them. The situation you describe may be whats happening Im not sure. They are recent CK purchases so I assumed the correct pieces are in them. But they do run very close to the plate. I dont think shimming a .4mm max gap between pin and backplate will do any harm and may help stop it striking in a crash.

They do run very well though.

Thanks, Yabby, if the CK and shaft came assembked, then the mismatching I described can be ruled out.