SLT-6 is 0.060 Over - Runs Hot...Why?????

-

mydart270

myDart270
Joined
Mar 18, 2014
Messages
190
Reaction score
7
Location
Brattleboro, VT
hi and hello:

I think I have a problem. My 1968 Dodge Dart 270 2DR HT with SLT-6/225 is running hot after 15-20 mins. Temp gauge starts at mid-point (normal) then goes right to RED ZONE.Temp will come down to mid-scale after I switch on heater. Temp might go up again depending on my driving. For the most part I am running very near to the RED ZONE when driving. Don't like this fluctuation and high-temp so I know I have a problem. Car has less than 1500 miles after rebuild and sat for almost 25 years because of corporate moves, no interest, and life's events. Totally restored, pretty, and a real charm but motor is the issue.

Motor was rebuilt in 1995 and builder said bored to 0.060 (should have said STOP and I need a new block). OEM rad re-cored, new 160 deg stat, and new OEM water pump. Timing -5 deg and dwell 43 deg. 50/50 anti-freeze installed, new sender. Motor fired up in 2020 (finally) with lots of pre-lubrication, runs very smooth I must say on the highway.

Question: is 0.060 too much so that minimum block mass to extract heat? An old MOPAR guy told me this is my problem so try a bigger radiator. Alternative is re-sleeve block. Motor is original from car and afraid to try a SL-6 after 1970.

Will tear into this in April after vacation in FL.

Thx for your thoughts and comments. I love A-Body Forum. Great experience and lots of help.

Love my Dart...
 
Possible airlock in the cooling system. Are you running a shroud? Is the fan stock? Condition of the water pump? I've had the impellers rust off.
 
Possible airlock in the cooling system. Are you running a shroud? Is the fan stock? Condition of the water pump? I've had the impellers rust off.
All of that said above and get a non contact thermometer ( HF has them ) and verify that the engine / radiator is getting as hot as the gauge shows, it could be a gauge - wiring issue.
When the car sat for the 20 years or so, did it have coolant in it? If so the radiator could be plugged and rotted internally. The thermostat may be blocked with debris that broke loose and contaminated the new coolant. BTW, a 160 F thermostat would not be my choice, these older engines do well with some heat in them. I would go with a 195 or at least a 180.
And on your original question, .060 out of the bores on a slant is nothing to worry about. Folks have taken them .100 over. I have a slant in my 68 Barracuda that is .080 over and it does not have over heating issues.
 
Last edited:
Car has done less than 1500m. Has it run hot the whole time, or is this something new?
 
I don't know what's makin it run hot but it AIN'T the .060" over bore. As John said, slant sixes have been taken out to .100 and beyond. They AIN'T a thin casting. From stone cold, start it up without the radiator cap. If the radiator is SLAP full, drain some out until the level is about 1" below the top of the tank. Let it run until you see and know the thermostat is open. You'll know this by the coolant getting hot and starting to circulate. Once that happens, reach in and goose the gas a few times. If it gushes coolant out of the radiator when you goose it, the radiator is stopped up and the coolant has no place to go.

It could also be a stuck thermostat. If the coolant in the radiator never heats up and it does not begin to circulate, the thermostat is likely stuck closed. Do you have a thermal temperature gun? Best 20 bucks you'll ever spend. Maybe 10 if Harbor Freight is runnin a sale. You can determine a lot of things up to and including verifying a stuck thermostat, measuring incoming and outgoing coolant temp from the radiator and other nifty things. Cheap as they are, this is a tool you should already have, especially trying to troubleshoot an over heating issue.

"I had the radiator re-cored" is no reason to overlook the radiator as a suspect. Remember, "new" and "rebuilt" do not mean "GOOD".

Oh and get that STUPID 160* thermostat outta there and install the correct 195*. The thermostat has ZERO to do with how hot an engine runs. It needs the correct thermostat OR no cooler than a 180*.
 
A new build often shakes loose rust from the water jacket. Then after a few trips, your radiator is plugged.
 
Definately verify temp gauge as I dont have alot of confidence is the sending units and calibration after all these years. Other issues I had with mine when I got it and it was running hot were: stuck heat riser, distributor weights, float setting (verify, dont assume rebuild is even close).
 
I wouldn't trust the factory gauge. My 63 shows hot on the dash gauge and correct temp on the mechanical gauge.
 
^^^ I changed the factory gauge sending unit twice and got the same result.
 
Changing the sender twice still doesn't mean its reading correctly. Too many of us have been fooled by this with wrong senders, bad regulators, pinched wiring, etc. Verify with another gauge. Is it boiling over?
 
Changing the sender twice still doesn't mean its reading correctly. Too many of us have been fooled by this with wrong senders, bad regulators, pinched wiring, etc. Verify with another gauge. Is it boiling over?

I think that's what his point was. That he changed the sender twice and it still read incorrectly. I might have misunderstood.
 
You understood correctly.:thumbsup:
Always gotta check with the poor quality of parts now-a-days.
 
Boring it .060" is no problem for a Slant, long periods of sitting can cause sludge to settle in the bottom of the water jacket. Hot tanking can often remove the sludge, but many times it doesn't, which diminishes water capacity inside the water jacket. Also, double check timing and carb jetting, both can affect cooling.
 
get that STUPID 160* thermostat outta there and install the correct 195*. The thermostat has ZERO to do with how hot an engine runs. It needs the correct thermostat OR no cooler than a 180*.

This needs a rethink and is unrelated to his overheating issue.


I never change multiple things at a time when trying to isolate the cause.

The timing, that's something that strikes me. "-5 at idle and dwell of 43"...So What's the power timing?
OP , set the total timing to 32-34 degrees@full advance and or +5 btdc to +10 btdc @idle then test drive it.

And make sure the exhaust heat stove flapper is not stuck closed!
 
Last edited:
Not the .60 bore. I'm thinking water circulation around the engine.

Flush the block properly, have the rad pressure tested and checked for flow at a reputable rad shop, Check the thermostat for proper opening by putting it in water and boiling it with a thermometer alongside it. ( 180 degrees if I recall ). And , as was suggested, check the water pump impellers as this engine did sit for a long while.
 
Are you really sure about the radiator?
So glad it is not the block bored to 0.060 oversize.

Radiator was re-cored by a reputable company as I retained the OEM number and Chrysler star on the upper tank.

Car always did this after rebuild.

I will ck out this problem more when I return. The common item in this issue is the gauge in the dash so will ck with another gauge plus IR gun when I return. A new 195 deg stat will also help.

Great feedback and recommendations to try. You guys are the best.

Thx again...mydart270
 
Just run with the cap off for 20 minutes at idle. if your temp is actually HOT. It will be boiling over. If its not boiling over its not HOT. New gauge or proper sender. Can also monitor the temp sender with a DMM and see what resistance its showing. If your impeller is rotted off, you should see little movement in the rad coolant once the stat opens (or lack of stat for testing purposes)
 
So glad it is not the block bored to 0.060 oversize.

Radiator was re-cored by a reputable company as I retained the OEM number and Chrysler star on the upper tank.

Car always did this after rebuild.

I will ck out this problem more when I return. The common item in this issue is the gauge in the dash so will ck with another gauge plus IR gun when I return. A new 195 deg stat will also help.

Great feedback and recommendations to try. You guys are the best.

Thx again...mydart270


Ok , but if after going through the other members suggestions and the problem persists, I would definitely re-check the rad. As others said, don't assume because you had the rad re-cored, it's working properly. Easy to make a mistake for anyone.
 
Car always did this after rebuild.

! Something isn't right. Compression check or better yet, a leakdown check. The best would be a two input remote T/C thermometer across the radiator, but not a lot of those around that are affordable.
 
If opening the flow to the heater core makes that much of a difference, I'd guess blockage in the radiator cooling circuit.
 
-
Back
Top