head gaskets

duke.johnson Fri Jan 11, 2013 11:44 am

I've read about adding head gaskets for higher nitro. Something like one gasket for 10%, two gaskets for 20% and so on. And Rusty just said something about more gaskets for cold weather and less for warm weather. Is there a standard? I usually run 30% but have never run more than one gasket.

**RknRusty** Fri Jan 11, 2013 12:16 pm

The rule of thumb is two shims(gaskets) for 20% nitro and add one for every 10% above that. Below 20% you can use one, but I say play it safe with two. You can blow a glow plug if you use too few. With 25% fuel I still use two, but with 35% I use three. This is with Cox heads. The replacement plugs with the clamp type head that Cox International sells(Merlin plugs and clamp rings) are higher compression than any Cox head and require extra shims, one or two extra. I never run less than three with those.

Cold air is more dense so just a drop in temp raises compression. I don't have any rule of thumb for adding or removing a shim at certain temps. As long as you follow the general nitro rule you should be safe from blowing plugs. In the other thread where you saw me mention temperature, I was using a high compression Merlin drop-in plug. I probably blew that one from cranking it while flooded on a cold day with three shims. Being flooded it put too much pressure on the glow element.

These...
http://coxengines.ca/cox-.049-glow-plug-adapter-extra-cooling.html
and these...
http://coxengines.ca/cox-.049-glow-plug-adapter-insert-style.html
...are the same thing. One just has more fins. They are higher compression than ANY Cox .049 head and require from one to three extra shims. Up to Five depending on how your engine behaves. I really like the three fin clamp ring. The five fin extra cooling one is mostly to look cool. head gaskets Cool

duke.johnson Fri Jan 11, 2013 1:15 pm

Rusty, thanks for the help.

crankbndr Fri Jan 11, 2013 2:43 pm

Its only the high compression heads that need more gaskets, you are fine running standard Cox heads with one gasket.
Thats most likely what you been running on 30% with one gasket.

John Goddard Fri Jan 11, 2013 2:47 pm

With the one gasket each 10% rule they start more easily
And the glows are less prone to blow.
Very Happy

duke.johnson Fri Jan 11, 2013 2:55 pm

crankbndr wrote:Its only the high compression heads that need more gaskets, you are fine running standard Cox heads with one gasket.
Thats most likely what you been running on 30% with one gasket.

Must be, we don't blow head very often. Would I get more RPM's with the high comp. heads? Is it worth the upgrade?

crankbndr Fri Jan 11, 2013 3:19 pm

You should be able to pick up 7,8,9 hundred RPM using the drop-in plug Rusty linked to, but they will need to be tuned like Rusty said.

duke.johnson Fri Jan 11, 2013 3:49 pm

crankbndr wrote:You should be able to pick up 7,8,9 hundred RPM using the drop-in plug Rusty linked to, but they will need to be tuned like Rusty said.

Seem like it would be worth it after the head was bought with the cheaper glow plugs. And by tuning, you mean adding or omitting head gaskets?

crankbndr Fri Jan 11, 2013 4:09 pm

Here is a Utube vid of a test run I did on my Mouse engine I sent to the tach race last year.
The first run is with a standard Cox head, the second is the Merlin drop-in. It picked up about 1K.
25% nitro, 5/3 prop

https://www.youtube.com/watch?v=eU6xQEkngkI

Paulgibeault Fri Jan 11, 2013 5:35 pm

Hi Duke,
Long story short...no there is no standard, with the exception that don't use less than one shim.
One of the reasons is because there is no standard prop & fuel. Different props & fuels have different head clearance requirements. Another reason has to do with piston to cylinder fit. A "looser" fit here seems to need less shims.
Changing from a standard Cox head to a TD head almost always results in more power/rpm (up to 1,000 more in some cases).

If you want maximum power, you'll have to burn out a few glowheads experimenting...
If your engine runs just fine with one head shim, I doubt adding more shims will make it run better.
Of course the real 'truth teller' is your tachometer, because it don't lie!

Lastly, some shims "fuse together" after awhile & look like just one when in fact there are two, giving mis-leading results. As well, it's not uncommon to have a replacement shim not seat flat in the cylinder.
Screwing on the glowhead with the thin copper shim scrunched in the threads usually causes a small leak that messes everything up. (happens to me frequently)
SOoo...feel free to play. Coxes are fun that way & sometimes the results are surprising.
Good Luck!

Cheers, Paul

duke.johnson wrote:I've read about adding head gaskets for higher nitro. Something like one gasket for 10%, two gaskets for 20% and so on. And Rusty just said something about more gaskets for cold weather and less for warm weather. Is there a standard? I usually run 30% but have never run more than one gasket.

pkrankow Fri Jan 11, 2013 6:38 pm

I have used zero shims with no nitromethane. Works fine. The aluminum head seats to the steel cylinder just fine.

No. Not recommended.

Phil

**RknRusty** Fri Jan 11, 2013 7:46 pm

I always like it when Paul G. tosses in on one of our topics. I can imagine him sitting back reading all of our follies shaking his head when he finally figures it's time to speak up and save us from ourselves. lol!

I have at least one Cox that I can't turn through TDC with no shims. It may have a stretched rod or something allowing it to contact the head. From the time I was a kid until I went full bore with my hobby a few years ago I had only ever used one shim, and only ever blew one plug. Maybe the old Cox fuel didn't have as much nitro as what I use now. Then one day I blew two back to back in the air on a Black Widow. That's when the RCG folks told me I should always run two. I have done so ever since then.

Mark Boesen Fri Jan 11, 2013 11:12 pm

....back in the day, lol, when i was flying 1/2a pylon a guy (who had been racing for a long time and was pretty good) told me he'd start with 4 gaskets and as the motor wore he'd remove gaskets till it got down to one and at that point the motor was pretty much worn out. Based on what Paul is saying it kinda makes sense.

Personally, i would set the compression based on tach and ability to needle.

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