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Old 11-07-2012, 05:48 AM
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305 compression testing

I have a 305 sbc in the basement and I was wondering, can I use a compression gauge and turn the engine over to determine if the rings are still good and that its got the right comp.. The bottom end is still in the block. i have a pair of good truck heads I can put on the block. Can I do it this way or does it have to be actually hooked up in a vehicle.
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Old 11-07-2012, 06:00 AM
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"Truck heads" are probably 350 heads with larger chambers.
Very low compression ratio on a 305. Use the 305 heads.
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Old 11-07-2012, 06:29 AM
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I guess what Im trying to say is, I want to know if the engine still holds compression. Make sure all the rings are good. Im trying to see if its still good so I can put it all back together or throw it away. So I was just going to bolt those heads on for now and hook the gauge up to each cylinder and test it. What should it read or how would I figure that out.

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As far as those 350 heads on a 305 Why wouldn that be ok. And what about 305 or 307 heads on a 350.

Last edited by allansmith859; 11-07-2012 at 06:35 AM.
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Old 11-07-2012, 07:30 AM
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305 compression testing

Most 305 heads have a 58cc chamber or smaller and small valves. The larger valves help with higher RPM flow. The 350 truck heads have 76cc chambers with valves as small as 1.72 & 1.5 to 1.94 & 1.5. Here is some examples. An 305 engine with 5cc flat top pistons and 58cc chamber heads will have a compression ratio of 10.0 while the same engine with 76cc chamber heads will have a 8.2 compression ratio. Another 305 engine with D cup 12cc pistons with 58cc heads will have a 9.2 compression ratio and with 76cc heads will have 7.7 compression ratio. As for testing compression on each cylinder by turning it over by hand IDK, but logically thinking I would not think you could spin the motor over fast enough to get an average reading.
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Old 11-07-2012, 09:22 AM
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A cranking compression test is good with the carb removed or wide open.

A Cylnder leak down of each cylinder will get more in depth analysis if you find the location of the leaking air.

The oil pump output pressure belive it or not, is a excellent indicator of remaining engine life.

If the heads are off flip them over and fill the combustion chamber with water. The shut valves should retain the water pool in the chamber.
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Old 11-07-2012, 10:38 AM
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OK Im getting confused. Im trying to see if I can use this 305 engine. Its the block and rotating assembly. The guy who had it said HE THOUGHT one of the rings was bad cause of a little smoke out the exhaust. That could be from a valve seal. I was asking if I could just bolt ANY heads on and take a guage, screw it into the spark plug hole and turn the engine over and see what it reads for every cylinder. If I do that what can I learn about every cylinder.
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Old 11-07-2012, 11:44 AM
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Originally Posted by allansmith859 View Post
OK Im getting confused. Im trying to see if I can use this 305 engine. Its the block and rotating assembly. The guy who had it said HE THOUGHT one of the rings was bad cause of a little smoke out the exhaust. That could be from a valve seal. I was asking if I could just bolt ANY heads on and take a guage, screw it into the spark plug hole and turn the engine over and see what it reads for every cylinder. If I do that what can I learn about every cylinder.
Yes just put a tea spoon of oil down the spark plug holes to make sure its got oil on the cylinder walls and you can use a starter motor to check compression turning over by hand will be to slow but may show any major issue. but really without the starter motor you will be in a crap shoot unless it reads perfect results. It will read very low if tunred by hand rings will have time to bleed down but that is normal for engine it only seals if its turning a little fast enough.

Smoke can be casued by bad valves seals usally. Ring too of course but seals usally go bad first. If no heads just check the bore for the cross hatch in the cylinder walls. If you have a lip in cylinder walls that cant be cleaned its usally needs rebuild. (Not Carbon but iron ground away)
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Old 11-07-2012, 12:42 PM
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The cylinder walls look great. There is a very little amount of surface rust that has appeard in two of the cylinders at the very top but I can clean that off. There is no lip at all they look very good. If I put these cylinder heads on this block which is bored .060 over with flat top pistons (head # 14101083). Would it be a good little street motor depending on cam choice and other applications??

Last edited by allansmith859; 11-07-2012 at 12:51 PM.
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Old 11-07-2012, 12:50 PM
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Quote:
Originally Posted by allansmith859 View Post
I have a 305 sbc in the basement and I was wondering, can I use a compression gauge and turn the engine over to determine if the rings are still good and that its got the right comp.. The bottom end is still in the block. i have a pair of good truck heads I can put on the block. Can I do it this way or does it have to be actually hooked up in a vehicle.
Don't see why this wouldn't give some sign of ring quality. The engine is probably pretty dry I'd put some lube on the cylinder walls before putting the heads on. A leakdown test may prove to provide more valuable information than a compression test if you've got a compressor you can use to presurize the cylinders.

Bogie
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Old 11-07-2012, 12:53 PM
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How do I do a leak down test??? What about puttin these heads on this engine if everythings good?? (14101083)
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Old 11-07-2012, 01:01 PM
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How do I do a leak down test??? What about puttin these heads on this engine if everythings good?? (14101083)
Leak down test is a air hose in the spark plug that pushes air into the cylinder at tdc and will let you hear if the air is leaking form crank case or intake or exhuast. Not really as useful if the heads are already off you can see any major issues.
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Old 11-07-2012, 03:25 PM
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Corvette heads are sought after you can sell them and buy a rebuilt set of 305 heads.

083 are 64cc heads normally hot 305's have 58cc heads. if its less compression its going to fall short on power for sure. But i am not much info on 305 engines i just toss then out of the way for something larger. But they can make some good power if done up right.

You may be able to find 58cc heads pretty cheap. They have small bore so many ppl over look them. I have used the heads before on larger engines to gain compression. But not a good choice for any major power upgrade. But will make a nice little street motor.
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Old 11-07-2012, 09:08 PM
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You want a set of 305 58cc 1.84/1.50 valves 1985 heads that hold water? If i can find them Pm me. You pay shipping.
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Old 11-08-2012, 12:15 PM
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So you are saying I cant use these heads for a stock 305 engine that has normal power.
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Old 11-08-2012, 12:32 PM
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it'll run.. but you'll be happer with 305 heads..
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