Pelican Parts Forums - Differences in diameter groups of pistons and cyls???

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Differences in diameter groups of pistons and cyls???

Andy's comments from this thread: http://forums.pelicanparts.com/911-engine-rebuilding-forum/388461-piston-gap-measurement.html

Quote:

Originally Posted by Eagledriver (Post 3718784)

The maximum allowable clearance is .004 inches. This is the wear limit. It can be measured with the old rings as they don't wear on the top and bottom, only on the edge that touches the cylinder. The top ring clearance is always the largest so if the top ring is good the rest will be good. The gap is largest at the 6 and 12 position.

I'm a big fan of re-using the old rings provided they are kept on the same pistons and cylinders. I've had brand new Goetz rings that had larger end gaps than the used rings that came off the pistons. I've used old rings on several of my engines with no problems. In addition if you use the old rings the wear limit on the ring land clearance is not as critical. It's new rings on pistons with excessive side clearance that cause problems.

You can buy replacement pistons but getting the proper running clearance is not easy. There are several different diameter groups for each size cylinder. You have to match the pistons to the cylinders by these groups. It's really best to keep the sets together if possible.

-Andy

What I am concerned about is I blew one piston and replaced it with similar weight group piston. Then had to replace 3 cylinders due to scarring of the sealing surfaces... I have the correct height group. Andy's comments, above, about matching diameter groups have me very worried now... and I JUST closed up the heads today :(

http://forums.pelicanparts.com/uploa...1201141582.jpg

Please don't tell me I have to tear it down again :( NOTE: I have a set of Henry's trick 906 replica air deflectors at the coater. I can put these in after the heads are on unlike the regular ones.

Best regards,

Michael

Pistons and Cylinder come in 2 cylinder height groups (as you know) and 4 diameter groups.
-Chris

If you look in a spec. book or the Haynes manual you will see the groups. They are marked 0,1,2, and 3. I can't recall exactly where the marks are but I think the piston marks are stamped in the face. Someone might help you with where to find the marks.

You can assume that the pistons you originally had were all the same size. Once you find the marks on the cylinders you will know if you have a problem. Maybe whoever sold you the one piston can tell you what group it was.

Sorry to make you worry.

-Andy

Crap... I guess I have to rip it apart again and make sure it all matches up :( I'll have to order more 574 as I've pretty much run out... Or can I simply remove the heads and cam carriers as one unit?

Best regards,

Michael

YEAH! All I have are '+'s on the top of my pistons... What luck! It is hard to see, but at full resolution it is apparent. But, what group is that? It is not the weight group as they are all '++' marked on the inside of the piston and I matched that correctly...

http://forums.pelicanparts.com/uploa...1201149408.jpg
(I'm soooo happy I took lots of pictures!)

Just checked the engine... looks like I'm in mismatch. #1 & #2 cyls show a '0' stamped in the same place the height group is stamped (logically the place to put the diameter group stamp). #4-#6 show a '1'. If I were a betting man I would now say I have two in group 0 and four in group 1 with all the pistons being in the same group. I can assume that since I had 5 pistons and 4 cyls from the original engine that '+' means group 1 for the pistons as that would match my group 1 cyls.

Here, it looks like, I have two pistons from group 1 put into 2 cyls of group 0. The specs on pistons do overlap for the two groups while the specs for the cyls do not. Now... what are the consequences of getting mismatched like this? Closer piston to cyl wall than designed means what?

Best regards,

Michael

PS I can't imagine the pain of trying to make a complete matching set from a box of singles...

To make sure you have a good set of matching pistons and cylinders it is best to do the following:

1. Weigh all the pistons with their wrist pins and make sure the spread in weight (heaviest-lightest) is less than 8 grams

2. Measure the top ring clearance on each piston to make sure the it is within spec

3. Measure the piston diameters with a micrometer across the skirt, below the wrist pin

4. Measure each cylinder bore diameter with a dial bore gage in (4) places (top and bottom of stroke at 90 deg orientation).

5. Make sure the piston to cylinder clearance (cylinder diameter-piston diameter) is below 0.004"

If you meet all these criteria, you've got a good set.

I actually measure the clearance directly with a feeler guage. If you take the rings off you can slide the pistons into the cylinders and slip a feeler blade between the skirt and the cylinder wall. The min clearance is toward the bottom of the cylinder. You need the blade in at least .5 inches or so to get a good feel. So push the piston down from the top over the feeler blade and see how hard it is to move. When it can just slip from gravity and doesn't rattle around I call that blade size the clearance.

Minimum safe clearance is .0015 inches. Wear limit is .004 inches. I like to have about .002 to .0025 if I can get it. I had a mismatched set last time I rebuilt and by measuring all the pistons in all the cylinders I was able average out the clearances. In other words match the largest piston with the largest cylinder and on down the line.

With your sets being close I bet you can match them up enough to be fine.

-Andy

Andy and others,

Since I have put the cam carriers on already can I simply unbolt the heads from the head studs, measure, and re-assemble in the same order if all is well? I assume, if I must swap cylinders around to get good clearance on all 6, I should strip the cam carriers off, clean them, and start over.

Best regards,

Michael

Ok, I checked the clearances. 0.0030, 0.0025, 0.0025 for #'s 1, 2, & 3... so all is good.

To check I simply took the heads and cam carriers off as a unit. Everything will go back together the same way so can I simply remount the head/cam carrier as a unit without unbolting the heads and re-sealing the head cam carrier surface?

Best regards,

Michael

Hey Michael... it sounds like you may be out of the woods on this one! I hope the best for you here.
Can you post a pic of Henry's trick 906 replica air deflectors? I did'nt know such existed and how actually do they differ from the stock ones?
Thanks for your help here and best of luck finishing the build with zero issues!!
Bob

Michael:

The engine looks great!

All your hard work is going to pay off. Even this rework that you are doing will pay dividends. Best of all, it beats the alternative, huh? Don't want to go down that nasty road again for sure. Cheap insurance as far as I am concerned, better safe than sorry.
Good luck!

Yes you can put it back together without removing the heads from the cam carriers. It makes it much easier. It's best to use new CE rings but I've re-used the old with success when I was too impatient to get new ones.

-Andy

All back in black and on to the next step :) Cam turns nice and smooth!

http://forums.pelicanparts.com/uploa...1202914392.jpg

Lots of cleaning to do today :) but, with luck, I'll be timing this evening or tomorrow.

http://forums.pelicanparts.com/uploa...1202914424.jpg

Thanks all!

Michael