GrahamC

I was messing about with some Cox 049 reedies today. I have an engine built from bits and pieces that was giving me fits until I tried shimming the cylinder. I used a papeer back plate gasket as a cylinder shim and the engie became much easier to start and ran more steadily. This small change made a difference of about 400 rpm. I am also using a Galbraith head and Nelson plug and will add/remove head shims too. I am just looking for an easy handling engine not necessarily a screamer.

I can leave the paper shim in short term but long term I would like to replace it with a metal shim. I would also like to mess about and try and find a "optimum" shim thickness.

So, the Cox 09 head gasket works but I haven't got any nor do any of the LHS's have any. Beside $9.99 USD is a bit much just for a metal gasket.

Any other sources?

I am going to try Milton Dickey method of making shims. Any other ideas on making metal shims?

cheers, Graham in Embrun near Ottawa Canada

combatpigg

My Feedback: (3)

Shimming the liner makes the exhaust ports open sooner, advantageous for high rpm operation. Combustion force is lost sooner, so torque is sacrificed. It sounds to me that you are looking for a way to make a healthy engine easier to handle, and that should only be done with head shims. Raising the liner also drops compression, but you are juggling timing towards what is optimal for high rpm and high nitro. Shimming the liner just a tiny bit to help orient the exhaust ports fore and aft is OK, it is good for every mode of operation, but the returns in performance aren't earth shattering. In short, lowering compression is a way to make some .049s more user friendly, but I think you would be better off taking a more direct route with head shims.

GrahamC

Good advice, thanks.

I was running this engine last night and did some more messing about with cylinder and head gaskets. Still more to try but I think without shimming the cylinder and adjusting the number of head gaskets I will hit a sweet spot. I already have 5 head gaskets and I am going to run some more tests +/ 1. Of course weather makes a difference and lately it has been stormy and humid.

This engine is pretty generic. It uses a car crankcase and crankshaft (the one with the taper nose on the crankshaft) a number 6 dual bypass cylinder and piston set, Galbraith head and Nelson heavy duty glow plug. Fuel is currently 25% nitro 25% castor. About 13,500 on a Windsor or MA 6x3 plastic prop.


cheers, Graham

combatpigg

My Feedback: (3)

You're in uncharted waters [as far as my experience is concerned] with 5 head shims. That seems like an awful lot, but what ever makes your tach happy, and your plane happy is all that counts.

GrahamC

when I first put the engine together it seemed odd that the piston was hitting the head with 1 head gasket, tried 2 and it still hit. I don't know if there is anything unusual about this piston / cylinder set. Needed three just so that it wouldn't hit and then a couple more to get the compression to where it seemed reasonable. There is also a lot of SPI on this set as well, one of the other reasons I was exploring the idea of cylinder shims. I recall reading somewhere (Dale Kirn perhaps) that .010 was about optimimum for SPI.

I will have to play some more as this is all interesting and educational but all I started out to do was put together a reliable engine for sport 1/2a control line combat type plane.


cheers, Graham

Toad

In the late 90"s, Cox did send out a bunch of engines that had this problem. The piston hitting the head. At my hobby store, I remember shipping back 12 engines. (all TD .051's) They sent me 12 good ones in return. It seems to me it was the cyl that had the machining error on the base where it seats to the case. I say this because others using the Black Widow's were finding the same issue. I suspect you have one of these cyl/piston sets from a bad run that escaped QC

Toad

These bad cyl's exhibited excessive spi also.

Toad

I would shim the cylinder up, untill you have .008-.010 spi. I am pretty sure the exhaust timming would be corrected back to original spec by doing this, and then you would only use the usual 1 or 2 head gaskets to make it run like it was intended too.

dieFluggeister

The Std Cox head will allow the cylinder to enter the head if the stroke is tall enough where the Galbreath will not. For some reason some, but not all, of my surestarts have this issue and I need to use at least one shim with the Galbreath or else the piston will strike at TDC.

GrahamC

Now that is an interesting tid bit of information. I am going to measure it to see if it falls into this oddity. This is a #6 cylinder which I think was intended for car use, I don't know if there was any difference between an "airplane" cylinder and a "car" cylinder.

In any respect the engine runs, gives reasonable performance on the Windsor 6 x 3 plastic prop although I will be using the APC 5.7x3 on the plane and will test run it before hand on this prop.

It has been an interesting diversion playing about this engine and I learned some things in process.

cheers, Graham in Embrun near Ottawa Canada

************
Did it a bit more research and discovered that this cylinder is a number 1

and as described by Joe Klause:

No.1. A two bypass cylinder with a large exhaust port that permitted sub-piston induction. It was originally made for the 290 and 291 series engines that were used in plastic cars later. it was used in the first Black Widow engines. At that time the top cylinder fin was not milled to accommodate a maintenance
wrench. Subsequently, the Black Widow I cylinder had two narrow slits for each I exhaust, and the top fin was milled. but I the cylinder was not numbered in this I configuration.

more details on cylinder numbers can be found here:

http://www.clstunt.com/htdocs/dcforu...ID1/17842.html

as previously posted in the Cox Reed Valve FAQ

cheers, Graham

combatpigg

My Feedback: (3)

Well I'll be a monkeys' Uncle! I never heard of that machining problem, that might explain why KK sold a really thick liner shim?

Jim Thomerson

Actually I found that Cox 049 crankcases varied in the height of the cylinder base. I found this sort of hard to believe although I made the measurements myself. I asked the late Gene Hempel (a Cox consultant/guru) about it and he said I was right. It was no news to him.

GrahamC

Thanks for all the input guys, much appreciated.

Well, I stripped down this engine and measured the cylinder against a known good TD cylinder, measurements where all the same, base to lower exhaust post lip, width of exhaust port etc. So it doesn't fit into the "machining error" description although I did find the story interesting.

What I did find was more than likely the source of my issue all along, the fuel line tube in the tank was kinked and sat on the fuel nipple in an odd sort of way.

I trued up the back of the crankcase (sheet of glass 1200 wet or dry and lots of oil techinque) and the top of crankcase where the cylinder seats so that the exhaust ports are fore and aft and polished up the crank and crankcase shaft bearing area. I had already done all this before, this was just refining all of these steps one last time.

Degreased and cleaned everything up, replaced the fuel feed line being much more careful when closing up the back end and finished reassembling the engine using no cylinder shim

I tested it this evening and am quite pleased with the result. Using 3 cylinder head gaskets, the Galbraith head and Nelson plug, the engine starts easily, two or three flips from cold - one of two when hot (that's no starter or using the spring); it needles well - postive and not too sensitive and using an APC 5.7x3 prop consistent and steady runs at 16,250 rpm.

Bottom line is, my pinched fuel line (inside the tank) was causing the bulk of my problems.

cheers, Graham in Embrun near Ottawa Canada

*** Forgot to add, the SPI is right around .010 ***

combatpigg

My Feedback: (3)

Nice work, Graham. It is amazing how close you have to look to see all the little things that can go wrong, this is a perfect example of why the .40 size and over guys don't like messing with our little toys!

bogateer

How can one tell if the piston is hitting the cylinder head? Will it be an obvious "bump" in the cycle? How many gaskets does it usually take to fix this? I have a NIB TD .051 that I's prefer not subject to an early demise.

Thanks,

Rob

GrahamC

I use a no more scientific method other than by feel when you turn the motor over by hand; there will be a definite "feel" when the piston touches the head.

One head shim is usually sufficient but you may find the motor is better behaved with 2; depends of course on what fuel you are using (nitro content), prop etc.

cheers, Graham in Embrun near Ottawa Canada