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Author Topic: Mark III Combat Special conrod  (Read 2074 times)

Offline Chad Hill

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Mark III Combat Special conrod
« on: November 22, 2013, 06:05:56 PM »
I've degunked four MK III Combat Specials I picked up in the back room of the local hobby shop. None would even turn over but now they're all free, have super compression and good bearings.

A couple of them have very worn conrods (why didn't Fox bush them?). I checked with Fox and they had no Mk III rods left in stock. Question: does anyone know if the Fox 40 (six-bolt back plate) conrod will work on a Mk III ? Or any other rod? Thanks, Chad

P.S.- Will the Fox 40 six bolt back plate gasket fit on a Mk III?
« Last Edit: November 23, 2013, 08:22:14 AM by Chad Hill »
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Offline kenneth cook

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Re: Mark III Combat Special conrod
« Reply #1 on: November 23, 2013, 11:19:39 AM »
               Hello Chad, I'm pretty sure all the gaskets are the same. Last time I sent Fox a e-mail, they had very few of those rods. A bigger problem however that happens is up top. Although your telling us the lower end is ovalled out, I would dig deeper and inspect the wrist pin hole in the piston itself. You will fix one problem and the other will definetly  pop up. If all is good terrific. I have several rods that are currently bushed. The rod itself has enough wall thickness to do that and I'm sure Bob Matte could assist. http://www.flyfreeflight.com/Site/Home.html     

I'm also curious to know whether or not your using the stock head button. I found this to be the most horrific head design ever made. It literally rips the coil right out of the plugs in all the engines I used it on. One run= one plug. I was using high quality plugs so it's certainly not a plug issue. I've hemi'd the head and added additional gaskets and found this to solve a good portion of the problem. Fox still sells the double bubble head for the .40 but it can be easily turned down to fit the .36 Ken
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Offline Chad Hill

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Re: Mark III Combat Special conrod
« Reply #2 on: November 23, 2013, 01:53:33 PM »
Thanks Ken, for the info and also the link to Bob's site. I didn't know about it and will check it out.

Yes, the rod lower end is indeed ovalled out on two of the motors. Didn't notice any obvious slop at the wrist pin holes so maybe I lucked out. The motors didn't look like they had been run very much but wow, a couple of them were really gummed up, inside and out. Seems like the original owner had stored them by soaking them in castor.

I've heard about that head problem and have been told that the hemied Mark 6 head will fit the Mk 3, and that Fox may have some still stashed away. Haven't called them about that yet, tho.  Not sure I could hemi the original heads myself without trashing them. Thanks for the tip about the 40 head.

Another problem I've heard about is that the lower inside diameter of the steel drive washer needs to have a radius ground into it to prevent causing stress cracks in the crank. None of my four motors had that mod and a couple of the cranks do have minor scratches where the washer contacts it.

Hard to believe Duke & Co. didn't foresee these things, having already been in the motor business for years. I was an ST guy back in the 70s, and these are the first CS I've had. Thanks for the pointers, it'll make things easier. Best, Chad
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Offline kenneth cook

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Re: Mark III Combat Special conrod
« Reply #3 on: November 23, 2013, 04:41:04 PM »
                      Chad,   Marvin Denny set me up with a properly fit drive washer. I did have the crank break on a flat out full blown run. I thought the engine was trashed when it went on the shaft run. It actually richened up and finally stopped. Just so your aware, the MK IV crank fits the MK III. This will however require a MK IV drive washer as the III has smaller splines. I've been able to hemi the head out carefully with a plug screwed into the stock head and chucked into a drill press. I was able to steadily use a X-acto and cut the head button. I sanded and polished it and you would've never guessed it was done like a caveman. I have one and one only shim that lifts the liner up. This works exceptionally well in combination with a Mccoy MC-9. I was using a 8.5x7 Rev-up that is somewhat getting difficult to find. These are extremely cool engines though and I hope you get things sorted out. Ken
« Last Edit: November 23, 2013, 06:07:13 PM by kenneth cook »
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Offline Chad Hill

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Re: Mark III Combat Special conrod
« Reply #4 on: November 23, 2013, 06:33:32 PM »
That's a great idea for hemi-ing the head, Ken! I may give it a try.  H^^
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Offline jim welch

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Re: Mark III Combat Special conrod
« Reply #5 on: November 23, 2013, 07:34:32 PM »
Chad...I got oem gasket and screw sets, bushed rods,head buttons of all kinds,hold down heads, backplates, cases, etc for the mk iii's enough to fix yours if ya need them.I got some more tigre's like I sold you on the auction a while back
also.If I can help you can send them to me and i'll fix them for ya.  Jim
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Offline Chad Hill

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Re: Mark III Combat Special conrod
« Reply #6 on: November 23, 2013, 09:38:58 PM »
Hello, Jim! I sent you an email. That was a nice C35 you sold me, I put it on a new Winder with a pacifier pod. ;)
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Offline phil c

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Re: Mark III Combat Special conrod
« Reply #7 on: November 25, 2013, 08:28:36 PM »
Quote from: kenneth cook on November 23, 2013, 04:41:04 PM
                      Chad,   Marvin Denny set me up with a properly fit drive washer. I did have the crank break on a flat out full blown run. I thought the engine was trashed when it went on the shaft run. It actually richened up and finally stopped. Just so your aware, the MK IV crank fits the MK III. This will however require a MK IV drive washer as the III has smaller splines. I've been able to hemi the head out carefully with a plug screwed into the stock head and chucked into a drill press. I was able to steadily use a X-acto and cut the head button. I sanded and polished it and you would've never guessed it was done like a caveman. I have one and one only shim that lifts the liner up. This works exceptionally well in combination with a Mccoy MC-9. I was using a 8.5x7 Rev-up that is somewhat getting difficult to find. These are extremely cool engines though and I hope you get things sorted out. Ken

A little safer way to open up the head is to use a ball-head bit, either 1/8th or 3/16th, in  a Dremel tool to do the trimming.  Make sure you can brace your hands and use a light touch so the tool doesn't start to vibrate.  Cutting metal with a knife, like they do wood turning is a bit dicey.
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Offline Mike Greb

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Re: Mark III Combat Special conrod
« Reply #8 on: November 26, 2013, 04:25:53 AM »
 On one fox head I used a 1/2 in grinding bit from my dremel tool kit and it produced a nice bowl, not very precision, but it got the job done.
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Offline jim welch

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Re: Mark III Combat Special conrod
« Reply #9 on: November 26, 2013, 04:31:43 PM »
Chad...making you some head buttons..sent you an e mail....jim
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