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"COX Tee Dee .051 on duty"



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Post  Cox International Thu Jul 14, 2016 10:45 am

We have sold thousands of our TD .049 HC glow heads with very few issues. However, we now have a customer who claims that, after installing our glow head, RPM dropped by 2,500, using the same fuel and propeller.

To note is that our glow heads are manufactured to Cox OEM specs, using original Cox drawings but we are also aware that Cox made different versions of combustion chambers; so far we have come across three different ones. They may make a difference in RPM, but 2,500???

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Post  crankbndr Thu Jul 14, 2016 11:05 am

If he went from standard head to high com he most likely didn't tune it by adding head shims?
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Post  JasonB Thu Jul 14, 2016 11:18 am

The high compression glowheads could allow an engine to become over-compressed. Nitro, oil, mixture, load, temperature and humidity all have an effect on just how much compression a glow engine will stand before before it is overcompressed.

With too much compression for a given combination of conditions, you get mucho heat, finnicky handling and a loss of power. Short plug life too.

It's long been common to start with 3 or 4 head gaskets under an HC head, and remove them until removing another doesn't improve performance. Put the last one back in and drive'er like you stole'er.

There's always the possibility that something has gone wrong in manufacturing, or that he has a poor glow-head seal, or..or..or.. But overcompression seems most likely.

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Post  Cox International Thu Jul 14, 2016 12:31 pm

Thanks for both replies and the customer went from an OEM HC head to our HC head.
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Post  Paulgibeault Mon Jul 25, 2016 2:10 pm

Yes, there does appear something definitely wrong here...but what? I have not come across this phenomena. I might hazard a guess that the replacement head
leaks. OR possibly a head gasket was "crunched" during installation (preventing a good seal). One way to check that speed fliers use is to squirt kerosene around the glow plug / fins & quickly turn the engine over. Bubbles indicate a problem.
Always difficult to diagnose over the Internet... Good luck!

Cheers, Paul



Cox International wrote:We have sold thousands of our TD .049 HC glow heads with very few issues. However, we now have a customer who claims that, after installing our glow head, RPM dropped by 2,500, using the same fuel and propeller.

To note is that our glow heads are manufactured to Cox OEM specs, using original Cox drawings but we are also aware that Cox made different versions of combustion chambers; so far we have come across three different ones. They may make a difference in RPM, but 2,500???

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Post  Cox International Mon Jul 25, 2016 2:27 pm


Thanks Paul and we did send suggestions to the buyer and have not heard back so far.

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Post  RknRusty Mon Jul 25, 2016 3:44 pm

Bernie, I'd forgotten you are having reproductions of the #1702 newly made. There could be possibly some difference in a batch of the platinum(plated?) elements. There are some meticulous members here who could run a comparison for you between an OEM 1702 and one of your new ones. Or a couple of new ones from different batches.

Paul had a good point too, maybe your disappointed customer could deck the facing of his plug on some 600-1000 grit paper glued to a flat glass pane... carefully and not aggressively.
Rusty


Last edited by RknRusty on Mon Jul 25, 2016 8:32 pm; edited 1 time in total (Reason for editing : Corrected a mistype)

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Post  Cox International Mon Jul 25, 2016 3:52 pm

Yes Rusty, the filament composition has been upgraded to a better version beginning the first time we made them but we have sold 1,000's of the beasts with very little issues.

No worries, the buyer has gotten some good info and, if he is still experiencing issues, I am sure we will hear from him.
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Post  Cribbs74 Mon Jul 25, 2016 7:44 pm

Maybe the customer is in error. It's not always a matter of just spinning an old one off and installing a new one.

If it is an error on the customer's part then he won't be happy with an even higher compression head! Very Happy

Oh well, the customer is always right! Or so i've heard.
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Post  RknRusty Mon Jul 25, 2016 8:36 pm

If I may ask; is the element a platinum plated tungsten coil or an alloy containing platinum? I think it varies by manufacturer, I only ask out of curiosity.

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Post  Cox International Tue Jul 26, 2016 11:34 am

RknRusty wrote:If I may ask; is the element a platinum plated tungsten coil or an alloy containing platinum? I think it varies by manufacturer, I only ask out of curiosity.

90% Platinum / 10% Rhodium (Cox used 90% Platinum / 10% Ruthenium)

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