So I have been having overheating issues on my 1971 cutlass with 455. Runs cool in the city, overheats on the highway. Have 2 row Champion aluminum radiator with 1'' tubes, Milodon high flow water pump and thermostat, and new clutch .. good shroud. At like 2200 RPM it starts to heat up and I have to get off the highway when it gets over 215 degrees. Based on this Im questioning my piston clearance. Reason being is that I had it bored over .040 which would put me at 4.166 diameter. My sealed Power 369P + .040 pistons show to be 4.165 diameter, which puts me at .001 clearance. Does that seem a little tight, or should this be sufficient? All of the right parts were used as far as I can see.

 

Seams to me that a good machine shop would not install pistons with only .001 clearance.
I'm more incline to say not enough air flow, or to much water flow. The water is not staying in the rad long enough to cool.
Do you have A/C ?

On your new water pump, did the impeller fit all the way to the edge's or is it smaller and a huge gap from the impeller to the edge ?

Gene

 

I was thinking you had a forged piston. Those are cast so I am not as concerned you have a piston clearance problem as I was before. If its truly at .001 clearance is too tight but I don't think it would be. Is there signs of scuffing on the skirt of the broken piston?

 

Its an AC car but no AC in it currently. The new Milodon water pump actually helped out, so i dont think its that. the old water pump made it get hot faster. I even went from a 180 stat to a 195 for the reasons you sugested(more time in radiator).. Regardles of any of that it still runs hot at higher RPM.

As far as the scuffing, I will need to remove that piston this weekend, but i will let you know what I find.

 

I have had 3 Milodon thermostats that were defective. They only opened about 70 percent. Check your thermostat.

 

What I was getting at is, with A/C, the condenser out front may be clogged with dirt and bugs and what not, will affect air flow.

Gene

 

It's certainly on the tight side of the factory tolerance which was .001-.002 for a non-W30 455. The "idjit" builders that did mine also went with a tighter fit, but i don't have the same overheating issues when i up the rpm's for awhile.

Also, i don't like what i see with the heads but i'll respond to your other thread for that.

 

Hi something else you may want to check is your piston ring gap



http://www.wiseco.com/PDFs/Manuals/RingEndGap.pdf

 

My speedpro cast are running at .005 as requested. 2323f's. No problems to date after 3 years. No slap. JFYI

 

Hot Rod Magazine said that the original factory fit for the 455 in 1971 was done with a .0015" feeler at a 3-12 pound pull at 90 degrees from the pin axis, at room temperature. The tolerance range was given as .001-.002" except for the W30. The range for W30 was .0025-.0035". Under "Blueprinting" (page 10) of the factory provided "Performance Tips From Oldsmobile Engineering" it says: "For all out racing internal specs and clearances should be checked. A very important item is piston to bore clearance. With production pistons it should be .003 to .004 inches." Other publications from Oldsmobile engineering have indicated values as high as .008-.009" when racing with forged pistons, as their expansion is more than production pistons.
I have a 1965 442 which is completely "stock" in the piston & bore. It has a tendency to operate on the "hot side" (any speed) during the summer months. While I have never measured the piston to bore clearances, perhaps it suffers also from being a bit too tight.
Often the piston manufacturer will recommend a piston to bore clearance. It probably should be followed except for extraordinary situations.
I'd check the operation of the thermostat with a accurate thermometer in a pot of water on a stove. The rating should determine the lowest temperature at which the engine will operate after warm-up.
With overheating situations be sure to check everything. At highway speed overheating, I'd be sure to check ignition timing, fuel-air ratios, and exhaust congestion, as a minimum.