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Discussion Starter · #1 ·
HELP!!! I've toasted 3 motors already and don't want to toast a 4th. But it did take the 3rd motor going down to find that my oil pan is being sucked dry! I have a 1984 350 Chevy block with 400 heads with everything stock otherwise. The motor is in a boat and at running 3000-3500 rpm after about 20-30 minutes completely empties the 5 quart oil pan. Had great oil pressure until the motor ran dry.

What is the problem? Is there a problem with 400 heads on a 350 Chevy with oil restrictions? I have no idea what else it could be.

Hope someone is able to help me. Thanks in advanced for your time.
 

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Have you checked out the oil drain back holes in the head to make sure it`s draining back? Are the ports open by the distributor and timing chain to assure oil drain back? 400 heads on a 350 aren`t great for performance, but they will work. There should not be any issue with oil drain back unless it`s being severly restricted by the head and lifter valley. I would think the problem is elsewhere as sucking the oil pan dry is not a very common problem.
 

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Discussion Starter · #4 ·
We rebuilt a 1981 350 Chevy block with the 400 heads twice. First time motor redone with rebuilt heads, all new bearings and new stock oil pump. Great oil pressure, real low motor temp then all of a sudden the oil pressure dropped and all the crank bearings overheated running 3000 rpm for about 30 minutes.

2nd time, same motor combo (1981 350 Chevy block with the 400 heads) with different crank and new oil pump. This motor lasted 12 hours before #8 connecting rod went out.

Thinking that perhaps the block had a cracked internal oil passage, the 3rd motor had a different 1984 350 short block that was running for 4 years. Took the 400 heads with yet another new oil pump. After about 30 minutes running between 3400-4000 rpm all of a sudden the oil pressure dropped again. I checked the dipstick immediately and there was no oil. I looked in the oil filler holes in the valve covers and saw some puddles of oil but not allot. After about 5 minutes, the oil was back in the pan.

Each time, the motor was completely disassembled, cleaned completely and put together properly. New OEM stock oil pump with 5 quart pan each time!!!

Any other ideas please?
 

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you say the engine is in a boat. Can you discribe the installation? I/O, V-drive, etc.

At speed, is the rear of the engine lower than the front? Is the crankcase vented well. Does the engine have much blowby at speed?
 

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Discussion Starter · #6 ·
Inboard 350 Mercury/Chevy, 22' open bow, 3900 lbs., Mercury Alpha 1 Outboard Drive. Engine is level, oil pickup 1/4"off the pan in the rear of the motor. Crankcase is vented through each valve cover into the top of the air cleaner. Did not notice any blowby whatsover. The oil just ain't draining back down into the pan. Any difference in oil drains between 350 & 400 heads?
 

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Shouldn't be any functional difference between 400 and 350 heads that would affect oil drainback that much. I run the 350 vortec in my 19' Baja at 5200 all day towing barefoot skiiers and never had an issue. A high-volume pump has the risk of pushing more oil than can drain back, but a standard pump should let you go WOT for hours at a time.

There should be no reason why this is happening, but try different heads? Or a different oil pump? Seems like they are the common factor in your builds. And... how are you determining that its because the oil pan is sucked dry? That's a bit of an urban legend, and it would explain your symptoms, but its unlikely and hard to diagnose. Are you using street tolerances on the bearings? Try opening them up to .0030 or .0035 and use 20w50. That's a more common marine/HD setup.
 

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You mentioned that after 5 min. the oil was back in the pan-How much oil was back in the pan? If enough was getting back to read on the stick, then you definitely have drain back restrictions. Double check the drain back hole/head gasket area with a flash light and make sure the gasket isn't blocking the hole somewhat. I've had to trim a few in the past, but its rare. Back in the day, some circle burner friends used to put pipe stem cleaners in the pushrods to slow down the oil flow to the top end, keeping it in the pan for the bottom end. Thats something you might try to help confirm your situation. I wouldn't recommend you leave the stem cleaners there, just try it for diagnostic purposes, then correct the real problem.
 

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Is your block vented properly? If it's not, and you get too much blow-by (which is produced at a maximum in a constantly loaded boat engine), the oil may have trouble draining back as air is being forced the other way through the head oil-drain holes.

I've had trouble before with the pan being sucked dry, but it was due to cold thick oil, a HVHP pump and a low capacity pan.
 

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Discussion Starter · #12 ·
Thanks for all the responses :)

Actually, nothing was reading on the dip stick when I turned off the motor.

The oil filter's on the block. It's vented as it originally was made with 1 hose out of each valve cover going to the top of the air cleaner. Will research this more on my motor.

Perhaps I have the wrong lifters and pushrods?

Bearing's were set .0025 with the proper SAE 30 needed for pushrod engines.
 
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