Thread: fresh rebuilt 350 overheating rapidly and extremely low oil pressure

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Okay everyone I am new to the club and I am in dire need of some help, I just rebuilt a 350 small block. I took apart my old block and reused the crank, rods, pistons, and heads (TBI heads) other than that everything is new. 1st thing I did was sent the new block to the machine shop to have it hot tanked, cleaned and inspected after I got the a okay from the machine shop I started the rebuild. The block is a 1973 350 bored 30 over. I installed new valve seals in the heads ground and ground the valves. Installed a new camshaft and bearings making sure the get the order correct and I clocked #1 at the 5 and 7 o'clock position and the other four at the four o'clock position from the top of the block. Installed a new Comp Cams camshaft part number 12-239-3, and new lifters. I plastiguaged the main and rod bearings as the old ones showed they were .020, everything checked out and I began the final install on them making sure to torque everything correctly according to the manual. Installed a new high volume oil pump new rings (making sure to separate the ring gaps. Installed a new water pump, thermostat and sent the radiator out for cleaning and some minor repairs. After getting everything together and double checking all the bolts and seals filled the engine with 10w-30 motor oil, cam break in oil and a new Hastings oil filter. Now on initial start up fixed a couple of small coolant leaks topped off coolant. The engine has extremely low oil pressure only 20 psi at 2000 rpms, around 5 at idle, and overheats immediately after start up temp just climbs until I shut it off thermostat is only a 180 degrees. Any and all help would be greatly appreciated this is not my first rebuild it is however my first one with this kind of trouble. Also I pulled the valve covers off and checked for oil flow I get good flow through about half of push rods but some are not oiling at all the only noise I am getting from the motor is a small amount of lifter tapping.

What's the timing set at? The Hi-Volume oil pump is a no-no with a stock pan, pumps too much oil and will pump the pan dry before the oil can drain back into the pan! Plasti-gauge is just not a good way to check bearing clearance IMO..... Did you check the crank journals with a micrometer at all, were the rods re-sized? (stock rods have a habit of going egg shaped, no way to get proper clearances with an egg shaped big end on the rod) Did you "burp" the cooling system? An airlock in the system will cause severe overheating!

Originally Posted by Dave Severson

What's the timing set at? The Hi-Volume oil pump is a no-no with a stock pan, pumps too much oil and will pump the pan dry before the oil can drain back into the pan! Plasti-gauge is just not a good way to check bearing clearance IMO..... Did you check the crank journals with a micrometer at all, were the rods re-sized? (stock rods have a habit of going egg shaped, no way to get proper clearances with an egg shaped big end on the rod) Did you "burp" the cooling system? An airlock in the system will cause severe overheating!

....and you mention you "...reused the crank, rods, pistons, and heads (TBI heads)", but then you say the block is 0.030" oversize. Were your pistons OEM or already oversized 0.030"?

wrong rotation water pump????????????


plastic guage??????? what bearing size did you use as you say your crank -.020


block 4.030????what size was your pistons??????


Too many under/mis stated conditions to determine your total condition

I went back with the .020 bearings as the that is what the machine shop said to use, I took the old ending block with the lower end still together to the shop as a whole. They checked everything and said that the lower end was quite reusable. this rebuild was just to try and gain some performance as the old engine was still running well. Went with a new water pump for a 1979 Chevrolet 350 k10. The old block was already bored the specs I gave are all from the old block as well as the new one both blocks have the same casting number on them. The timing is set at 8 degrees BTDC with a timing chain and gears set at 2 degrees retard, I was told that would give it a little more low end as was the whole point of the rebuild it just had way to much top end and no low end power. Since the post I pulled the thermostat it was stuck closed, removed it all together to see if that would alleviate the overheating did not stop it just slowed it down, I am still running the original clutch fan, wondering if that is not either the problem or part of the problem, I also changed the oil and filter gained 10psi on the oil pressure initially, then when i brought the truck down to an idle only had 5 psi again. I am considering using a thicker oil 10W-40 and changing the brand of oil filter currently has a Hastings on it. I have heard good and bad about them, then again I have worked 20 years in the parts industry and have heard good and bad on just about every brand out there. Also I will switch it back to a standard volume oil pump. Thanks for all the quick suggestions please keep them coming.

water pump is for a stock 350 1979 Chevrolet K10, yes sorry for the misspelling Plastic guage, crank had .020 bearings and so did the rod bearings, both blocks 4.030, pistons 4.030 Hope that can help you help me

Originally Posted by jerry clayton

wrong rotation water pump????????????


plastic guage??????? what bearing size did you use as you say your crank -.020


block 4.030????what size was your pistons??????


Too many under/mis stated conditions to determine your total condition

Oils with more viscosity or pumps with higher volume are not going to fix the problem of excessive clearances in the engine. If you have too much clearance on your mains, rods & cam maintaining pressure is a losing battle. Dave told you initially that a high volume oil pump and a stock pan is a recipe for disaster. You can suck the pan dry before oil can have time to drain back down from the upper end, starving your pump. Did you put in an extra capacity pan, baffled to keep the pickup primed, along with the high volume oil pump?

Are you running a PCV, and if so do you have a breather in the opposite valve cover to relieve the suction pressure of the PCV? Strange as it sounds, that alone can affect your oil pressure.

And just to be clear, what the guys are saying is that PlastiGage is not a reliable measurement method for bearings. If your journal or bearing has any deviation from being perfectly round the PlastiGage will give you a bad reading for the spot where you place it. You need to use inside/outside micrometers, measuring multiple positions on each journal/bearing to know your actual clearance.

Keep the dialog going, and I'll bet that you'll get to the bottom of your problem pretty quickly.

Roger and Dave I pulled a dumb one and put the stock pan on it not even thinking. Roger I had the crank and the block to the machine shop at the same time and had them order the bearings for me to install, now I did not check them with a micrometer before I installed them due to me trusting my machine shop (knowing all to well to error is human). Roger as to the PCV and Breather part, yes to both I have a PCV valve on the left valve cover and the Breather on the right valve cover. I believe we have the oil pressure cause found and that is installation error on my part , I just do not understand the overheating part of it all. So if we can continue on that part, my thoughts are the low oil pressure for whatever reason is causing to much friction in the motor and causing it to overheat? However I do wonder if the there are other causes that I am over looking and can the heater hoses being reversed cause it to overheat?

Heater hoses can't affect your cooling except in extremes. Your heater valve might not open against reverse flow, but the only impact would be no heat. I'd say that you have air trapped in the cooling system, which will block coolant flow. Some Jeep models are known to trap air and have to be nose high to get the air out, either parked on a steep uphill slope or jacked up to an extreme. Later models had a 1/2" pipe plug in a high point in the radiator hose, and factory instructions were to fill, start the engine and remove the plug to allow all air to escape, then quickly put the plug back in place and run the engine at 3000rpm for a minute or two to purge any remaining air. I always drill a ~1/8" hole in the t-stat plate to let the air out of the block during fill, but you mentioned you've pulled the stat so there's no restriction there.

You mentioned the cooling fan clutch. The thermostatic clutches are known to fail over time, but you should be able to hear if it's engaged or not by the roar of the extra load. I can't say this is right, but from another forum:

The way I diagnose a fan clutch is pretty simple. Start with the car off. Jiggle the fan forward and back. If there is any play at the clutch shaft, its bad. Give the fan a spin. It should move freely, but stop almost instantly, as if the clutch were filled with mashed potatoes. Start the engine. The fan should turn pretty briskly; almost as fast as the crank. Now use a rolled up newspaper to gently try and stop the fan. You should be able to when its cool. Throw away the ruined newspaper Rev the engine, you should notice that the fan increases speed, but not proportionally to the crank speed. The clutch is full of viscous fluid, so the air friction on the fan starts overcoming the friction of the fluid in the clutch and it stops accelerating with the engine. The fan won't roar. Then let it get hot. You might have to drive it around some stop and go streets to get it to lock up the clutch. Now when you acclerate, you should hear the fan roaring a little louder. If you stop the car, shut off the engine, and try to spin the fan, it shouldn't move.

A bad fan clutch won't come into play quickly. If you're seeing a rapid temperature increase you've got a flow problem, IMO.

Do you have an air mover // large fan you can place in front of the radiator?

Just guessing you put the fan shroud back in, and any fan spacer.

Is the fan clutch operating correctly?

( edit )
Completely agree with above post

Check your new water pump, could have a slipping impeller or a reverse rotation impeller (seen both).

Heater hoses reversed won't cause an overheating problem. All the heater core is is a small radiator, once the cooling system is full of water (and no air pockets), it doesn't really matter to the engine temperature which way the water flows through the heater core. As T-Top mentioned, a good shroud is critical too cooling and also where the fan is placed in relationship to the shroud, ideally the middle of the fan blades are even with the opening of the shroud. Removing the T-Stat isn't a good idea, if the flow of coolant is unrestricted through the engine it can flow too fast to do any cooling, put in a quality T-Stat!

Originally Posted by 36 sedan

Check your new water pump, could have a slipping impeller or a reverse rotation impeller (seen both).

Mine had the back cover bolts hand tight out of the box. We found those during a dyno run

Well it looks like this one is becoming a tear down and do over. While I tear it down what do I need to look for as signs of causes for the low oil pressure and overheating. I have no idea how tell if I have a reverse impeller on the water pump can any body post some pictures of what I will be looking at on the water pump, are there any parts that might be reusable like the new cam and lifters or should I just start a new with all the issues I have had. Also since this seems to be a start over should I start a new thread for advice on the next build.

When you put the head gaskets on, did you make sure the holes in the block for water were the same size as the head gasket? The same goes for the manifold gaskets. If the gaskets were not matched to the holes in the block, that can lead to lack of flow.