[Retlt0123]

1969 350 pontiac motor with #47 heads, I recently assembled this motor with new head gaskets, melling spc-7 cam, new lifters and push rods. when i took compression readings with no intake on the block and the motor was of course cold i got the following readings. cylinder #1 90 psi and it went to 120 psi after squirting some oil in cylinder. Cylinder #3 120 psi, Cylinder #5 120 psi, Cylinder #7 60 psi went to 90 psi with oil squirted in cylinder. this was on drivers side of motor
On passenger side Cylinder #2 120 psi, Cylinder #4 120 psi, Cylinder #6 120 psi, Cylinder #8 110 psi that went to 120 with oil squirted in cylinder. My questions are as follows :

1) is 120 psi normal for this motor Mileage unk ?

2) is it possible that only the rings are bad on cylinders 1, 7 & 8

are there any other reasons for this weird pattern of readings ?

[pastry_chef]

Compression readings can in theory also be affected by valve timing. For example if you have a few cam lobes on the way to their demise or bent pushrods. Your improved results with added oil in the cylinder would tend to suggest bad rings though.

For more detailed diagnoses, you would want to perform a "leakdown test" with the required equipment. Basically you pressurize the cylinder and measure how much leaks from it in percentage. Also when this test is performed, if you listen carefully you can often hear where the leak is. For example bad rings will hiss from the crankcase, a bad valve will hiss from the intake or exhaust as the case may be.
If two side by side cylinders are bad it could be a blown head gasket.

[pastry_chef]

120 lbs compression will not make for a peppy street motor. Performance pump gas compressions should be in the 160 to 180 ballpark.

You said you assembled, does that mean the motor has never ran?
You installed new rings? If so did you have the cylinders honed?

[Retlt0123]

i should have been more specific but it's a long story here goes :I bought this motor from someone who said it was a running 350 from a pontiac tempest. when i received it i discovered the motor had a mismatched set of heads on it and no oil filter housing and holes in the oil pan. The member i bought it from refunded me the money for the engine however i was still out the $100.00 i paid a guy to go get it for me and bring it to me. I have since found a used set of matching #47 heads that were off a running motor and cleaned them up and put new vlve stem seals on them. i bought the new cam and push rods and lifters and found a used oil filter housing. I bolted all these parts onto what i thought was a good bottom half only to discover the compression readings i posted. Since my post i have removed all the bolted on parts and removed the pistons from the cylinders that showed low compression, I was hoping it would just be a ring problem and i would simply re-ring the offending pistons and be ok. However i have found that the #1 and #7 pistons were broken in that the lands between the first and second compression rings were cracked off in pieces. I found that the 2nd compression ring on piston #8 was seized in the groove and not free spinning and had no gap at all. I know what i should do ( have engine bored 30 over and buy all new pistons and rings ) however at this juncture it is not what i can afford to do at this time. I have bought a new ring set and was considering honing the cylinders and finding 2 used pontiac 350 pistons and putting new rings on all cylinders and going from there.

If i hone the cylinders will this cause me to have to buy different rings ?

I am really getting depressed here as i have spent way more money on this engine than i had anticipated for a "Running 350" I don't know what to do as i have found that even if i get this motor together and running it still wont mount into the 1981 firebird i bought it for as this motor only has the old 2 bolt motor mounts and would require special mounting kit from PY or Ames. I am about to give up and just sell all the parts. i don't know ?

Thanks for letting me vent

[pastry_chef]

You will not need to buy new rings if you hone the cylinders lightly.
The purpose of the hone is just to remove any possible cylinder glaze and provide a pattern that will help the new rings seal.
I trust you have no "ridge" at the top of the bores? If so you would want to remove the ridge as well.

You are right.. boring would be better, but you can patch it together to make due. Hopefully there are not any other nasty suprises to ruin your plans.

[ctgross]

What type ring set did you buy? If you are just looking to build a driver that won't be beat on, you should consider a ring set with a cast iron top ring face. They will seat and conform better to a cyl. that is slightly out of round vs a moly chrome. Hastings Co. has an excellent tech. sec. on their site that would help you on your build. www.hastingsmgf.com

[Tim Corcoran]

If the bores don't have any deep scratches a light hone and new rings should bring her back to life. I would go for it.

Tim C

[Retlt0123]

Thanks for the replies, but i have decided on the following:
I am having the block bored .030 and the crank will be ground .010. I have purchased new pistons and rings in .030 over and new rod and main bearings in .010 . i have never gone this far into a motor before, but all that is left to do after i get the block and crank back from the machine shop is to reinstall the crank and main bearings and put on the new rings and reinstall pistons and rod bearings. I think i can probably do this without screwing it up.I have my fingers crossed. thanks for all the help on this site and on this post in particular i have learned much from this forum and look forward to the day i can put this motor in the car and fire her up. It should be an interesting combination 1969 350 in a 1981 firebird . i still have motor mount problems to overcome in order to make this work, but i will also figure that out in time.

thanks to all who have helped
Steve

[johnta1]

Make doubly sure you clean the block and crank good.
Use a lot of hot soapy water.
When you can wipe the bores and lifter valley with a white paper towel and it comes out with no residue, you are close.

Clean the crank oil holes with a rifle bore brush. (or similar)
Wipe it with the paper towel until it shows no residue also.

[smuryof]

I want to add, I just finished my first rebuild last month, and all the small stuff (thorough cleaning, exact torque specifications, clearances, the whole 9 yards) can seem like redundant overkill sometimes, but let me tell you from experience, after you have it all together and installed in the car, you're going to wish you did all that for reassurance. Checking for ring gap in three places in each cylinder, for each ring, plastigage on every bearing surface with stingy scrutiny on being in-spec - when you hear strange noises later on when you're driving this, you don't want to find yourself wondering "Was that plastigage reading on #3 actually tapered?" etc. etc.

FWIW..

[pastry_chef]

Well now that you have the bottom-end apart, I'll suggest something you may not be aware of. I do realize this may not be in your budget.

BOP's viton rear main seal. It replaces the at times problematic rope seal that is factory. Everyone I know of that has.. loves it.

http://www.bopengineering.com/beltdr..._topend2.shtml

One more.. if you are not 100 percent on your assembly knowledge.
http://www.boxwrench.net/product_index.htm
http://speedtalk.com/engine-rebuilding-dvd-1.html

Very cheap insurance, compared to what a mistake could cost!