how to id high compression olds 350 eng

scarborough

Master Mechanic
Sep 30, 2016
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I have a olds 350 that's painted gold not sure of the year, I read a article that said the olds gold engine is a high compression motor with a 10.25 rate. just wondering if anyone knows if this true. and what casting number would be stamp on the heads
 

69hurstolds

Geezer
Supporting Member
Jan 2, 2006
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That only works if you KNOW the engine has its original paint and original 4 barrel carb. 2-barrel carb versions I believe were not high compression either, but 9:1 isn't bad for the low-comps. I can make a gold 455 appear in about an hour if I really wanted to. Look for the casting numbers. Leopard can't change his spots, but he can paint over them. But the 350 started appearing in blue I think in 1973. Not sure.

IIRC, the best chance of a 350 being high compression is you would look for a "2" to start the casting number on top of the block above the timing chain area. If it starts with a 2 and ends with an 8, you're in the sweet spot of 68-70. GM used a 2 to start the casting number and was used up until 1976, so you still aren't always out of the woods.

If you see a 3A or 5 at the start of the casting number, forget it. It's a later 350 and 77 and up had the displacement tattooed on the side of the blocks, and they were definitely low compression.

Pistons and head chambers MAKE the compression. So even if it's not a high compression engine to start with, you can make it one with the right choices.

5 or 6 numbered heads are likely the best, 6's having slightly bigger exhaust valves, and W-31s getting 2" intakes. Again, in that 68-70 year, but you can get by with 7, 7-sub-A or 8s (7-sub-A's are not to be confused with the 7A 307 smog heads) from the 71-72 time frame as well. If you see A.I.R. tube holes on top of the exhuaust ports...just...no.
 
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Oct 14, 2008
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The #6 heads have the same 1.56" exhaust valve as most Olds 350 heads. I measured 2.07" intake valves, added by the previous owner but reused the stock measured 1.56" exhaust valve on my current #6 heads. Aren't the 7 or 7A a large exhaust valve? The #8 has a 1.625" exhaust valve but the bowl is very shrouded and make them pointless stock. Unless it has 5 or 6 heads or the pistons were changed, it will be 8.5 to 1 at best. As said, even some of the early 2 bbl were around 9 to 1 compression. A borescope is your friend here.
 
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69hurstolds

Geezer
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Jan 2, 2006
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I recall reading years ago somewhere the 68-69 350 heads have smaller exhaust valves. That's what I was alluding to. Maybe that's just some BS. I don't know.
 
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Oct 14, 2008
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My understanding is the 330 and 350 valve size was 1.87" intake and 1.56 exhaust except the W31 1.995 or 2" intake and 1.625" exhaust. The #8 are 1.87" intake and 1.625" exhaust. The 72 7A heads may be the same. The 3A are crack prone and have an OK intake port with 1.87" valves. The small 1.5" exhaust valve and shitty exhaust port is a choke point on all later number and large A SBO heads.
 

scarborough

Master Mechanic
Sep 30, 2016
309
66
28
That only works if you KNOW the engine has its original paint and original 4 barrel carb. 2-barrel carb versions I believe were not high compression either, but 9:1 isn't bad for the low-comps. I can make a gold 455 appear in about an hour if I really wanted to. Look for the casting numbers. Leopard can't change his spots, but he can paint over them. But the 350 started appearing in blue I think in 1973. Not sure.

IIRC, the best chance of a 350 being high compression is you would look for a "2" to start the casting number on top of the block above the timing chain area. If it starts with a 2 and ends with an 8, you're in the sweet spot of 68-70. GM used a 2 to start the casting number and was used up until 1976, so you still aren't always out of the woods.

If you see a 3A or 5 at the start of the casting number, forget it. It's a later 350 and 77 and up had the displacement tattooed on the side of the blocks, and they were definitely low compression.

Pistons and head chambers MAKE the compression. So even if it's not a high compression engine to start with, you can make it one with the right choices.

5 or 6 numbered heads are likely the best, 6's having slightly bigger exhaust valves, and W-31s getting 2" intakes. Again, in that 68-70 year, but you can get by with 7, 7-sub-A or 8s (7-sub-A's are not to be confused with the 7A 307 smog heads) from the 71-72 time frame as well. If you see A.I.R. tube holes on top of the exhuaust ports...just...no.
thanks for info. I have the engine stashed away in the corner of the garage, I will check to see what the casing number. just wondering if its a high compression motor.
 

spidereyes455

G-Body Guru
Mar 6, 2013
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Northeastern PA
To my knowledge only the 68-70 4bbl 350's were high compression. Having 5 or 6 heads is a good starting point but those heads were also used on the low compression 2bbl engines as well. Both flavors were also all painted gold until at least 73 if I remember correctly. They only way to know for sure is to either know with 100% certainty that it is an original unmolested 4bbl engine or to pull the heads and look at the pistons.
692cf1a9_734f_4a58_b3ab_32b8dd85728c_13d70386556c190a8809979bef26320b8a23ddef.png
 
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spidereyes455

G-Body Guru
Mar 6, 2013
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Northeastern PA
Also all the #5 and #6 heads had the same size combustion chamber regardless of application. Which I believe was like 64cc or so. The only difference in the heads between the 2 and 4bbl engines was valve size. Unfortunately again the only way to verify that is to remove the heads and measure the valves, there was no identification markings on them to differentiate between them.
 
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