Thread: 350 rebuild piston issue

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I'm currently rebuilding a sbc 350 with casting number 10054727. I'm trying reinstall the pistons with new rings and they are very hard to move once the compression rings are in the cylinder. The only way I can slide them further into the cylinder is by hitting it with mallet. The piston ring gaps are correct (checked with a feeler gauge) and the pistons are original to the engine. Every video I have seen of pistons being installed, the pistons can almost be pushed in by hand. Any ideas as to what I'm doing wrong?

I am not your expert here but you are not putting these in dry are you?

Should have some good grip, but a rap with the mallet handle should be all you need. I like some cheap 10W40 for assembly lube, on pistons.

Did you clean the piston ring grooves? Any build up in the grooves will prevent the rings from compressing completely.

Watching too many videos????????

Did you square the ring in the bore to check the gap?

Are the rings for oversized bore?

Remove top ring to verify its top ring problem, then remove other rings and try to install with just top ring to verify that it is top ring problem

AND, as already posted by 36------did you clean out the ringgrooves-they get a carbon buildup behind the ring as time wears on---------

And does the ring rotate easily in the groove??


And I am assuming that you removed the top ridge and honed the cylinder wall?

Welcome to CHR

OK riffraff, now that you have our attention, first question that has already been addressed by Jerry is: did you ridge ream the cylinders?
https://www.youtube.com/watch?v=79i-_-u3HzA

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Hi guys. Thanks for responding. I have the correct ring size and I checked the gap and it is within spec. Yes, I reamed the cylinders. I will try cleaning the ring grooves with a scraper to see if that helps.

Sorry it took me a day to respond, I've been in the garage all day.

For a scraper you can use a section of an old ring-just sharpen the end to make it sharp? and rotate arounf the grove-if you get black stuff out its probably the reason why--------

I scraped out the ring grooves and I did get a lot of carbon out but it didn't seem to do anything. I installed the piston nearest the back of the block and that went in as easily as before... I had to beat it in with a mallet. With that piston installed and torqued to spec, I could still rotate the crank shaft with much difficulty. When I installed another piston, I couldn't move the crank at all.

Is this even remotely normal? I still need to be able rotate the crank to install the rest of the pistons.

No, this is not normal. The piston once in the cylinders should only require light tapping if any to move them, and you should be able to rotate the crank with al cylinders loaded, although a large wrench may be required to turn the crank.
Check your ring grooves again, when cleaned properly you should see shinny metal at the bottom of the grooves, very important to get all the carbon out. Also check the ring gap in the cylinder as Jerry suggested being sure the ring is square in the cylinder by pushing the ring down slightly with a piston top.
You either have a build up in the piston ring grooves or the wrong rings.

Here is a summary of all of the responders questions. Answer them BEFORE the next step:
1. Will piston slide into its corresponding bore when there are NO rings on the piston? (yes/no answer)
2. Do you know how to tell if a ring is upside down? (yes/no answer)
3. Did you square the compression ring in bore with the piston and measure the end gap? (yes/no answer)
4. Did you do this measurement with ALL of the other rings for that piston? (yes/no answer)
5. How much did you cut off the end of the ring? (measurement answer)
6. Did the block get the ridge removed? (yes/no answer)
7. Did the block get honed with the proper crosshatch? (yes/no answer)
8. Did the pistons get the carbon removed from the ring grooves using a sharp tool (not a wire brush)? (yes/no answer)

Until you answer ALL of these questions, we are still on step 1 of the troubleshooting process.

Post the exact part number of the ring set your using -and I mean to include the last 3 numbers----------

and take off the top ring to install a test fit on the other rings, then reverse to just the top ring to verify that its a top ring problem---------

can you see the end gap of the top ring when its in the hole? should be aprox .020 gap visable


sorry robot------probly typing at same time

1. Yes, the pistons will slide into their bore without the rings
2. Yes, I followed the diagrams on the box they came in
3. Yes, I squared the ring in the bore and found the gaps to be in spec (.016 for second and .020 for top ring)
4. Yes, I measured the gap on all the rings
5. 0.0, all rings were in spec out of the box
6. Yes, I reamed the cylinder ridge during teardown
7. The block hasn't been honed.
8. Yes, I removed all the carbon in the ring grooves so that you can see the metal at the bottom of the groove

The rings are part number SUM-133-139-00

You may be confusing cylinder hone with boring. The cylinders need to be cross hatched for the rings to seat, this is sometimes called glaze breaking. It can be done with a hone attached to a drill motor.
Assuming the honing has been done along with all other procedures and you have liberally oiled the piston and rings prior to insert. The only thing left is how much force is required.
How are you turning the crank?

THANKS, Rif, this is a good start to confirm exactly what you had done so far. Too many times, the answer is hidden in an unanswered question.
Good point about the liberal oiling and the breaking of the glaze.
In the old days, I typically saw 50ish ft lbs to rotate the entire setup including pistons, all torqued to specs. Some would be a little tighter but I never saw a motor hit 100 ft lbs unless I had a main bearing/crank straightness problem. I suppose a too-tight rod bearing could contribute to the problem but it sounds as if the piston/rings/bore is the culprit. Lots of oil on the rings/piston skirts. We used to use the freebie STP for this but it attracted too much lint from the shop towels so we stopped.