rebuilt 350 with 3000 miles, 90 psi per cylinder??

[Ratt643]

Ok folks, here goes. I rebuilt this SBC last August. There was no ridge on the cylinders, so I used a glaze breaker(320 grit) on all the cylinders. Got a nice cross hatch pattern. I installed moly rings (.030 oversized) and put the lower end back together. Installed a new cam (very mild). The heads looked fine, so put them back on. Did a break-in with 30 wt Non-detergent oil and a quart of Rislone Zinc adiitive. Drove the truck to Detroit for the Woodward Dream cruise( 1200 miles round trip). I have driven it around town some but a few weeks ago, it snapped a rocker stud. I pulled a compression test before pulling the rocker covers. #4 was dead from the snapped stud, but all the cylinder read 85-95 psi on the compression test. The plugs are slightly oily. I put a small amount of oil in the cylinders today and pulled another compression test. all cylinders went to 100 to 110. I know the rings should have seated immedatly but the engine did not use a drop of oil on the initial trip and back. now its starting to use oil. Can this be the heads (bad valve guides) causing the valves to move in the guides and open up the positive seals(oil on the plugs) or did the rings not seat? The heads are a stock 882. I don't have a leakdown tester but can probably borrow one. I have a new set of World heads and RPM airgap intake, but don't want to put them on a bad lower end. Thoughts?

[maxwedge]

.030 rings, so we can assume this engine had been bored the same amount at some time, correct? The low compresion is internal so you need to do a leak down test for a proper diagnosis. Oil consumption can be caused by worn valve guides.

[Ratt643]

Yes, the engine was punched .030 and I went with forged pistons. I also lined up the ring gaps per the instructions.

[NHRA]

Originally Posted by Ratt643

Yes, the engine was punched .030 and I went with forged pistons. I also lined up the ring gaps per the instructions.

hopefully you mean they aren't lined up?

[silicon212]

Originally Posted by NHRA

hopefully you mean they aren't lined up?

To Ratt643:

Explain what you mean by 'lined up'. NHRA is thinking most likely what I am, that 'lined up' means you have all gaps aligned. You cannot do this - the gaps have to be staggered.

When I assemble pistons into an engine, I think of the 'peace symbol'. Line your oil control ring components so that the spreader comes together at the 'top' of the 'peace symbol' and that each scraper ring is 120' separated from that (i.e. the spreader comes together at the top of an imaginary equilateral triangle, and each scraper forms one of the two other points). Now, the lower compression ring is oriented so its gap is in alignment with where the spreader for the oil control ring comes together. The top compression ring is 180' from the bottom, forming the rest of the 'peace symbol'.

This is done to maximize compression and minimize leakdown due to the rings. A leakdown test, as noted above, is really the only way to determine the cause. When you rebuilt the engine, did you perform a valve job, or use new valves and have the seats properly ground, or did you reuse your old cylinder heads with nothing more than possibly changing the valve guide seals?

[silicon212]

[maxwedge]

Some 2cnd rings can be put in upside down, also.

[Ratt643]

Sorry if I confused everybody. the rings are aligned according to the insrtuctions that came with them. The gaps are not lined up. They are close to the way Silicon212 described. As far as the heads go, I did a quick lap job on the exhuast valves, as I had a machine shop face the top of the stems. I did install new positive seals before installing the heads. The engine backfired at high rpm some time back, which made think the valves are not seating and losing compression.

[silicon212]

That would be my guess as to the low compression - consider putting on good rebuilt heads with the proper springs, or do a full valve job to restore compression (and vacuum, performance and economy).

[Ratt643]

I bought a set of World heads with a fresh valve job and a port matched RPM Air-gap intake to put on this engine. I figured the heads were the problem but never hurts to get other inputs. I didn't want to put all this work into something that I was going to have to pull out and start all over again. I think I will hook up an air hose and see if I hear air coming from the intake or exhaust. Thanks for all the inputs!

[maxwedge]

Yo need to do the leak down test or you will not what is happening here, Double the work if the compression loss is in the short block.