How to clone a Cox Venom - RC Groups
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Nov 04, 2011, 12:24 PM
xenalook's Avatar

How to clone a Cox Venom

From Paul Gibeault, another fine guide. This one lists the parts required to clone a Venom engine:

We have all the parts in stock except the stunt tanks, which should be available next month.

At which point we will be assembling and selling a “Venom II”

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Nov 04, 2011, 08:29 PM
Registered User


How much for Venom 11 ?? You must have good idea since you are selling and making them. Thank you,Dan
Nov 06, 2011, 10:50 AM
xenalook's Avatar

Our plan is to retail price them @ $79.

Nov 07, 2011, 01:26 PM
Registered User
Bare's Avatar
49OCYTD is an exhaust slit sawed out Surestart cyl. according to the online catalogue.
No genuine TD cyl equals marginal improvement IMO.
In late 60's Kids retrofitted TD cyls to black widows.. routinely. Usually the very first 'improvement' made.
TD's were not overly popular then, having a reputation for being a PITA to live with/use.
The pricings clearly reflected this, as TD's were typically available for $12/13 new, whereas Black Widows were approx only $3 less.
Last edited by Bare; Nov 07, 2011 at 01:32 PM.
Nov 07, 2011, 01:34 PM
xenalook's Avatar
That is correct but the porting inside is 100% identical to the #4 Tee Dee 049 cylinder that Estes/Cox has been putting into their TD engines since aprx. 1996.

Not only did we get some of those TD engines and checked; we actually received about 400 of the TD cylinders. The only difference to the Surestart ones was the #4 stamped on them as well as a large open port that was cut slightly lower to induce SPI.

Nov 07, 2011, 04:57 PM
Registered User
My #4's and #5's have two boost ports per transfer port and a tapered bore. That is a total of two transfer ports and four boost ports.
Nov 07, 2011, 05:21 PM
xenalook's Avatar
Those are the old (Pre-Estes) ones.

The taper will likely add to performance but 4 boost ports are not necessarily better than two, albeit that myth being repeated for decades. Below is what Larry Renger (former Cox engineer) has to say about this:

We found the two boost port version ran higher rpm than four. The reason is that it introduces a swirl to the mix that enhances combustion. Four ports are too symmetrical. This trick was used in the Killer Bee, Tee Dee and Venom. I picked up the hint from the “Two Stroke Tuner’s Handbook”, a long out of print 2-stroke bible.

Anyone need the "Handbook" we would be happy to provide a link.

Nov 07, 2011, 05:58 PM
Registered User
The pre-Estes numbering system is, IMHO, the system that many continue to use.

The #4 cylinder classically has 2 bypass ports, 2 boost flutes per bypass port (4 in total), tapered grind, milled top fin, large exhaust port and SPI. The Tee Dee .049 I'm holding has the number 4 stamped on the cylinder and meets all the above criteria, except the tapered grind which I can't verify at this time.

The modified Surestart cylinders likely come closest to the #3 cylinder modified for the Venom, i.e., two bypass, one boost flute per side, SPI, no taper, large exhaust and milled top fin.

My only concern in using the modded Surestart cylinders is tied to how much grinding was done to remove the burrs from the exhaust port area. If the cylinder was finished only to the top of the exhaust port, then top end compression should not be affected, but crankcase pressure could possibly be impacted, reducing the volume of mixture passed into the cylinder during bypass.

Nov 07, 2011, 06:16 PM
xenalook's Avatar

The grinding likely has no effect whatsoever on engine performance, except to increase it (we hade them made in Israel).

3rd. party tests have shown this cylinder to perform within a couple of hundred RPM of the OEM Tee Dee cylinder; which is why we market it as a TD cylinder.

FYI, Cox manufactured thousands of TD cylinders stamped with the # 4 that had only 4 channels in total instead of 6.

What I am trying to say is that Estes continued with the old # system. Actually, Cox made 7 different Tee Dee cylinders in total and we still maintain that some of the newer versions outperform some of the older ones.

The Surestart cylinders are actually quite aggressively ported and perform better that the old Black Widow cylinders, for instance.

There is a caveat, however. Due to the increased tolerances of later years, engines can perform 20+% different depending on whether one gets a good or bad match.

Some are so well-matched that the engine overcompresses and won't start and others have dismal compression.

Nov 07, 2011, 06:23 PM
Registered User

Venom 11

All in all sounds good. If it has real stunt tank I'll get one to play with. Purdy looking engine.
Nov 07, 2011, 07:51 PM
Registered User
Originally Posted by xenalook

The grinding likely has no effect whatsoever on engine performance, except to increase it (we hade them made in Israel).

That's the most encouraging news I've heard.

Due to the increased tolerances of later years, engines can perform 20+% different .......

This is not surprising at all --- I have several boxes of Surestarts, some of which are quite good and some qualify only as parts engines.
I'm good with all the other component parts, including the porting --- the rework and deburring of the cylinders was my only concern. I'll probably give it a go on a build.
Nov 08, 2011, 04:58 AM
Registered User
surfer_kris's Avatar
The deburring is a problem to me. I guess there really isn't any good way of deburring the cylinders after the original honing has been made, without seriously affecting the rest of the bore. The deburring is only done up to the top of the exhaust port with what appears to be a regular stone in circular motion. You then get something that is very sloppy below the ports, while still catching at uncompletely removed burring at the remnant exhaust slits. I have not run mine yet (bought it as a spare) but it catches when turned over by hand, it is feels like acombination of uncomplete deburring and catching at the created step in the cylinder above the exhaust port. I don't know if I have been unlucky with my set or if they all come this way, I only bought the one set before it was correctly described on the webpage. To me there is a clear difference between these and the original TD cylinders. Perhaps a much longer and careful run in than usual, can polish out the step and roughness in the lower part, I don't know.

Here are a few images to illustrate the issue:
Nov 08, 2011, 08:24 AM
Registered User
Matt, with EX Model Engines, is carrying a piston/cylinder combo that I don't find listed on Bernie's site. It's a dual bypass double slit exhaust, has one booster port per side and the piston has been lightened and shortened for SPI.

I don't know how restrictive the dual slit exhaust is, but I doubt that this combination has been re-ground. The lightened piston paired with SPI might give better performance than the modified single exhaust cylinder combo.
Nov 08, 2011, 10:06 AM
xenalook's Avatar
Matt should answer that himself but it is my understanding that it is identical to what we offer; he just describes it differently lol.


The piston should not "catch" and, if you purchased it from us and provide us with your address, will gladly send you a replacement. We have had great success with these cylinders.

Nov 08, 2011, 10:08 AM
xenalook's Avatar

On a reed valve engine the SPI set will perform better that the one with the opened port, correct.

However, me market the open port one for rotary-valve engines and the SPI (twin-slit) version for reed valve engines.


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