[prostreet64]

As a 'measure twice, cut once ' guy, and in the case of engine builds never assume anything, I have to admit that I made a discovery on Friday as I began looking over the rotating assembly from the Bird that something was way off. As I posted several weeks ago we pulled the engine to do a basic refresh and find the source of the water getting into #4 cyl. That issue I am pretty sure was a head gasket leak. But when we were removing the pistons I thought I could see more of the top comp. ring than one would expect. Anyway, got the all the pistons and rods out and then did some measuring. The cylinders were at 4.165, right where they should be and the cross hatch looked perfect. However, when I measured the pistons which were supposed to be 40 over they turned out to be 20 over! We have been running this engine with those pistons since 2010 and I mean running hard. Why all the rings weren't broken I don't know. Never heard any piston slap and the car ran great. Those pistons were in a box marked correctly but I should not have assumed they were correct. Just installed the rings and never looked back. Sometimes it is amazing what you can get away with but I will not make the same mistake again.

[25stevem]

Wow, amazing!!!

Those .040” over rings where not supported by much ring land it would seem!
Have you looked for any witness marks for where the pistons may have been rocking enough to kiss the head, or the edge of the exh valve in the pistons valve notch?

I assume these are not cast piston’s because I can’t see how that type of clearance would not be heard, and I would assume cast pistons would be shattered by such!

[turbo69bird]

Originally Posted by prostreet64

As a 'measure twice, cut once ' guy, and in the case of engine builds never assume anything, I have to admit that I made a discovery on Friday as I began looking over the rotating assembly from the Bird that something was way off. As I posted several weeks ago we pulled the engine to do a basic refresh and find the source of the water getting into #4 cyl. That issue I am pretty sure was a head gasket leak. But when we were removing the pistons I thought I could see more of the top comp. ring than one would expect. Anyway, got the all the pistons and rods out and then did some measuring. The cylinders were at 4.165, right where they should be and the cross hatch looked perfect. However, when I measured the pistons which were supposed to be 40 over they turned out to be 20 over! We have been running this engine with those pistons since 2010 and I mean running hard. Why all the rings weren't broken I don't know. Never heard any piston slap and the car ran great. Those pistons were in a box marked correctly but I should not have assumed they were correct. Just installed the rings and never looked back. Sometimes it is amazing what you can get away with but I will not make the same mistake again.

This time you’ll put .030 pistons in it. Lol 😂 sorry couldn’t resist just kidding. It is amazing what one can do to an engine and it survive and then a simple small stress riser can kill one also. It’s really a crap shoot. My buddy built an engine w all his junk stuff from his old race engines. Bunch of cobblers together Mia matched stuff just to race through a season . The thing was the best engine he’d ever had .

[turbo69bird]

Probably had reduced drag on the assembly the new .040 pistons will probably make less Hp too bad you hadn’t dumped it would be interesting to see the difference . This is the type
Of thing old class racers would love to see data on

[prostreet64]

turbo, you may be right but I would think the pistons rocking back and forth so much as they were in our particular case would create drag? Who knows, maybe I should put 30 over ones in and see what happens, lol. Anyway, we plan to go 60 over to clean up the cylinders; all that rocking caused striations along the cylinder walls in a pattern about 1.5 inch wide at the top and bottom of the bore for the full length of piston travel. The striations are not deep enough to feel but they are clearly seen. Other than that I do not see any evidence of any piston to valve contact as Stevem had asked about. The pistons are forged by the way.

[Gary H]

That's incredible Gates! I would think that engine would sound like a 1960's diesel with that much piston to wall clearance.

[prostreet64]

Gary, I would have thought so too. I know the exhaust is pretty loud and the hyd. rollers are not exactly quiet but even so...

[AG]

Did you assemble the motor?

[dataway]

When I think about the engines I threw together as a kid ... not a torque wrench in sight, ball hone, new rings (never checked the gap), put in the bearings and never checked a clearance, liberal use of hammers and adjustable wrenches .... beat on it for 20,000 miles and never have a problem.

Nice loose engines with a good oil supply are hard to kill

[455dan]

Bought a decent running 1967 4 door executive an old lady bought new and had rolled over to 110000 miles on the 325 HP 400.
Thought it would be a good engine to just freshen up for a GTO "short" an engine.

No one had ever been into it according to the owner.
We got it apart and sure enough it was as built, with the original std. bore cast pistons.

Only surprise was when we measured the bores all had worn over .035 and a couple more were worn .047".

Machine shop had to take that block all the way to 60 over to finally clean all the bores.

How that engine ran and sounded so good is beyond me.

The other surprise was the crank, It was perfect just needed a micro polish.

[Dragncar]

Where did you measure the pistons, top or skirts ?

As a high school-er we did our share of back yard, ball hone, valve lapped engines and they all ran fine. A schoolmate used a brown paper sack as a gasket under a oil pump once. Cheap pointer tq wrenches. One kid hand sand papered his bores as he did not have hone.
All ran fine.
I rebuilt a 68 Fiat 4 cylinder as a kid, re used cam and lifters. The solid lifters got mixed up, no issues.

[Dragncar]

Originally Posted by 455dan

Bought a decent running 1967 4 door executive an old lady bought new and had rolled over to 110000 miles on the 325 HP 400.
Thought it would be a good engine to just freshen up for a GTO "short" an engine.

No one had ever been into it according to the owner.
We got it apart and sure enough it was as built, with the original std. bore cast pistons.

Only surprise was when we measured the bores all had worn over .035 and a couple more were worn .047".

Machine shop had to take that block all the way to 60 over to finally clean all the bores.

How that engine ran and sounded so good is beyond me.

The other surprise was the crank, It was perfect just needed a micro polish.

Witmore told me that 67 was the last year of plain cast iron on both top and 2nd rings from the factory. Once they went to single molly rings it greatly helped the wear problem.

[Mr Anonymous]

It can be surprising how wrong something can be and still work. I just saw yesterday afternoon a twin cylinder Kohler magnum 20hp, with .020 over jugs, one 0.020 over piston and one standard! Ran a decade that way in a piece of power equipment.

[sdbob]

Amazing. I thought I was backyard. When I built my first 389 back in 69. Using plasti gauge as I didnt have mics. Bought a click craftsman torque wrench. On a side note just bought a 2062 cubcadet,with a not running 20 hp command kohler engine. No holes in block. Thinking cam.

[Scott Roberts]

That's amazing it didn't rattle under load...

[25stevem]

When Pontiac changed over to Moly rings they went with double Moly, not single.

[455dan]

Originally Posted by Dragncar

Witmore told me that 67 was the last year of plain cast iron on both top and 2nd rings from the factory. Once they went to single molly rings it greatly helped the wear problem.

That makes a lot of sense. At the time, just assumed that particular block was just a bit soft- maybe a bit short on Nickel or carbon.

Got to say it was pretty obvious once that first head came off that she had some major piston to bore clearance issues .
he he

ps. It did go back together with Molly's

[prostreet64]

Well its time for true confessions. In post #11 when Dragncar asked if I measured the piston at the top or the skirts I thought, well I measured both. Problem is that my ignorance of piston design got me into trouble. I have never had a cause to measure pistons before now. I do my own assembly and double check all clearances, file fit rings etc., but the machinist does his part and then turns it over to me. Somewhere in the back of my mind I knew the top of the piston had to be smaller to allow for expansion but that is where my 'knowledge' ended! Then I made the mistake of measuring the bottom at the wrong point. Since I don't have an outside mic that will go past 4" I just grabbed my cheap caliper figuring it would be close. Since the rod was still attached I measured across the bottom in line with the wrist pin not knowing that this diameter would be narrower than the diameter at 90* to the pin. That dimension came in at 4.143. Using a cheap caliper and adding user error I figured that the piston must be 4.140 or 20 over. I can't get a good reading at 90* to the pin using my caliper so I ordered a mic that will go out to 5". It may well be that the maximum diameter at the bottom, 90* to the pin may be 4.160 which would be correct for the bore. What I don't understand is why there seems to be excessive rocking in the bore. It is greater side to side but even up and down is more than I remember seeing in any other build I have done. Well, I guess my miracle engine may not prove to be the case after all but at least I am pushing back my frontier of ignorance. Don't beat up on me too much.

[Dragncar]

Hey, if you do not do this stuff everyday sometimes you need a refresher. When I was file fitting my rings using one of my pistons to square up the rings I kinda tripped for a second when top top of the piston seemed very loose in the bore.

And this engine has .006 piston to wall anyway (hard block). So I flipped the piston over skirt first and the fit was much better. Then did a little research to confirm where pistons are measured.
Are you still planning on boring the block or just hone and new rings and old pistons ? You could hard block it and rigid hone it the extra .002 needed and end up with nice fresh bores.
Don't worry about speaking or making a post too soon. I have done it, life goes on.
Every single one of us have made mistakes even though some might not want to admit it.

[prostreet64]

Dragncar, thanks for the slack. The block is already filled to the bottom of the freeze plugs. I will need for the machinist to look at the block and determine how much we need to bore to clean things up. My guess is that we will probably need to go with new pistons. Besides the striations in 7 of the cylinders, #4 where water got in, is glazed and the cross hatch is essentially gone.