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post #1 of (permalink) Old 01-02-2002, 10:14 AM
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Burnt hole in Piston, Why?

I have an 85 CJ7 with 258. Head shaved .030, large values, ported and polished. Cylinders are .030 over. Running Jacobs Electronics Torquer. Weber 32/36 carb.

Mechanic just told me that the blow by is from a hole burnt through one of the pistons on the spark plug side of the piston. Mechanic suggests the following causes: (1) running too lean for too long, (2) predetonation, but often will occur in all cylinders and often will be white. That was not the case here is was dark. (3) spark is just too hot. (4) Run at high speed too long with some of the other issues above - 75 miles/hr is about the maximum on the interstate, but for 4 hours.

I would like to avoid this problem with my rebuild........SUGGESTIONS? THANKS IN ADVANCE.

Mindbender 85CJ7 I258 [img]/wwwthreads_images/icons/smile.gif[/img] 40 head ported polished [img]/wwwthreads_images/icons/smile.gif[/img]
Jacobs [img]/wwwthreads_images/icons/smile.gif[/img] 31" [img]/wwwthreads_images/icons/smile.gif[/img] 999 insulated hard top
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post #2 of (permalink) Old 01-02-2002, 10:32 AM
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Re: Burnt hole in Piston, Why?

What your mechanic told you has been correct. There are few other reasons I can think of:

1. Carbon build up not being removed and causing hot spot to form on the top of the pistion.
2. Flaw in the pision.
3. Electrode blowing off the spark plug affixing itself to the pisiton and causing a hot spot.
4. Valve material doing the same as in item 3.


1. Keep the fuel mix correct, not too lean, not too rich, but just right.
2. Use the correct heat range of sparkplugs (splitfires are famous for creating hot spots).
3. Keep the enigine timed correctly through all RPMs.
4. Keep vacuum advance in correct operating order.
5. Make sure the spark gap is correct for the applicaiton.
6. Use fuel line cleaners ( I like Seafoam) to clean out the combustion chambe ever 10,000, miles, or so.

These steps go a long way to keeping pinholes and hot spots from occuring.

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post #3 of (permalink) Old 01-02-2002, 01:21 PM
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Re: Burnt hole in Piston, Why?

I don't agree with his 'mechanic'....

* (1) running too lean for too long,*

Yup, that will do it. Lots of reasons for that, and you almost always get some warning that there is a problem.

*(2) predetonation, *

No such thing as "PRE-DETONATION".
There is Pre-Ignition, when some hot spot in the cylinder ignites the charge mixture too soon.
There is Detonation, when the charge mixture detonates uncontrollably instead of burning in a controlled manner.
Usually caused by cheap fuel, too high of compression ratio, too much load, too lean charge, too hot of heat range plug, ect...

I'm not real sure what you are trying to say in this next part...

*but often will occur in all cylinders and often will be white.*

IF the spark plug electrode insulator is white, that cylinder is too lean, the spark plug electrode insulator should be a light caramel color.
(With the hole in the piston, it's going to be black no matter what....)

*That was not the case here is was dark. *

IF the spark plug electrode insulator is dark brown, you are just a little too rich.
IF the spark plug electrode insulator is black, you are way too rich.
(With the hole in the piston, it's going to be black no matter what....)

*(3) spark is just too hot.*

NO SUCH THING! (especially with a Jacobs ignition)
It would take a direct plasma discharge to the piston top to burn a hole in it.
The timing would have to be set to TDC for the ignition discharge to even come close to the piston.
This is another one of those old wives tales...

*(4) Run at high speed too long with some of the other issues above - 75 miles/hr is about the maximum on the interstate, but for 4 hours.*

2 minutes could be too long if you had some of the other issues...

First of all, you need to determine if the piston hole was caused by...

1. Melt through. If so...
a. Is it a plain melt through?
b. Did some foreign material get to the top of the piston?
c. Does there appear to be a defect in the piston material?
d. Does it look like a nice neat hole, or like someone was trying to weld on the top of the piston and burned through?

2. Physical contact.
a. Can you see cracks on either the top or bottom of the piston around the hole?
b. Are the spark plug, valves or anything else bent in the combustion chamber?
c. Are there any signs of something running loose in the combustion chamber before bonding to the piston?
d. Is there a mark in the combustion chamber directly above where the hole is?

Get us some answers, and maybe we can help...

The most common causes of burning just one piston are... (in no particular order...)

1. Leaning out the idle mixture with out moving the idle screws together.
One side of the intake manifold will be way rich, and the other will be way lean.

2. Too much advance, or poor rotor phasing, spark not reaching the cylinder at the correct time.
If the spark is not getting to the cylinder at the correct time, the cylinder is up to it's own devices to fire the charge mixture. (Detonation, Pre ignition, Compression Ignition, Ect.)

3. Hooking up your PCV valve to a cylinder intake runner instead of the carb common vacuum port.
That will lean out a cylinder (or cylinders) in a huge hurry!
Always use the large common vacuum port on the carb base plate for PCV valve. The PCV is a controlled vacuum leak, and if you hook it to a intake vacuum source, the cylinder(s) fed by that intake vacuum port will be lean.

4. Having long duration, high lift cam, "ported and polished" heads, oversize valves on a basically low RPM engine.
All these things work against you at low RPM.
Large opening, long duration cams allow the port velocities to nearly stagnate before the intake valve closes at low and lower mid range RPM (where most people do most of their driving).
Larger, slick ports and valve pockets and bowls allow the fuel droplets to settle out, leaning the charge mixture at lower RPM.
The turbulence created by the cast iron or cast aluminum surfaces causes a 'boundary layer'.
That turbulence layer helps keep the fuel droplets from attaching to the metal surfaces and running off a liquid fuel, (liquid fuel doesn't burn, it pollutes your oil and my breathing air).
Liquid fuel doesn't burn, so your cylinders are lean....

5. Clogged carb primary jets, emulsion tubes, tube vents, or vacuum passages can all cause a lean condition at both high and/or low RPM, and coast or load conditions.
(Some monkey drilling around on jets or emulsion tube vents is a GREAT way to mess things up in a hurry!)

6. Vacuum leaks.
A vacuum leak around a runner, on in the intake to head gasket will lean out just one cylinder.
Cracks in the intake or head will lean out just one cylinder.
Unnoticed holes, vents, or ports that were not plugged will cause a lean condition... All of these are vacuum leaks.

So many cats, so few recipes...
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post #4 of (permalink) Old 01-04-2002, 10:28 PM
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Re: Burnt hole in Piston, Why?

Thanks, TeamRush. Almost more than I can digest in one reading. Anyway, I will take you advise and check out the burn. Timing, turbulence, hopefully no cracks, are the problem. I also took the ERG value off, but will be reinstalling that also. I will let you know what else I discover. Again. Thanks, WES

Mindbender 85CJ7 I258 [img]/wwwthreads_images/icons/smile.gif[/img] 40 head ported polished [img]/wwwthreads_images/icons/smile.gif[/img]
Jacobs [img]/wwwthreads_images/icons/smile.gif[/img] 31" [img]/wwwthreads_images/icons/smile.gif[/img] 999 insulated hard top
post #5 of (permalink) Old 01-05-2002, 12:57 PM
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Re: Burnt hole in Piston, Why?

I'll second the notion that the "porting and polishing" of the intake can work against you. For a low end torquer like the 258 this is a big no-no! By opening the runners you will slow the intake charge (venturi effect) like TR said. At low RPMs you can have liquid gas actualy dripping into the cylinder. This would cause hot spot real quick. I'd bet that the cylinder that burt has the longest intake runner (farthest from the carb).

If your running above 3500 rpm the vacuum is sufficent to keep the intake velocities high enough to allow the fuel to stay suspended. That's why race cars go for huge ports and valves with long duration cams. In town driving is a world away from this RPM range.

If you want to get more low end, you need to narrow the intake passages and avoid any sharp coners or dead zones in the runners. This will increase the velocities. If done correctly you can actually get a "ram jet" effect and really pack the fuel in there (atomized fuel).

To some up, what works for the street and strip crowd does not work for the off road crowd.

My name is Ted, and I'm a Jeep-a-holic. [img]/wwwthreads_images/icons/laugh.gif[/img]
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