.100 deck-piston?

Hey guys. Measuring up this 75 455 before I complete teardown. No matter what hole I measure, pistons are .100 down the hole. Is that not unheard of? Seems pretty far off the standard. 050 I read about. Haven't taken the bottom end apart, but I don't see the bearing being beat that much. Standard bore, so I'm doubting it's been apart. Any ideas? Thanks.

 

The later engines have taller deck heights. I’ve seen it many times on the 350s.

 

I got similar measurements with a later (75?) 455 I disassembled. This is one of the reasons those really low compression 455s are such inefficient, low powered engines.

 

Ya, that is one of those 7 to 1 compression boat anchors they built right at the end of the production run. I have driven a few, amazing how little they resemble the same motor, just a few years before. Even the mid 70's motors were strong compared to the last couple years. The 70 and earlier versions were rockets, in comparison.

Deposit the pistons in the nearest recycling bin immediately..

Keep the block, crank, con rods, timing cover if undamaged, oil pan and pickup, and the fasteners and oil slinger out of that engine, and throw everything else away. It's either worn out, or was no good to begin with. Even the water outlet on those is a flimsy steel piece, compared to the earlier cast ones.

Plenty 0f much better sets of heads out there for cores to rebuild.. or go alum if your really looking for performance. Same for the intake, get an early iron, or aftermarket dual plane for most applications..

JW

 

Thanks for the replies. I'm building this for a customers car, have him some options and he would basically like to go for a stock-ish 70 build. Going to get some cast replacement heads from T/A (along with basically everything else). But here's another question: Am I really going to be shaving .060+ off this block? Are there other pistons with improved pin height without breaking the bank on customs?

 

Autotech will customize your pistons without additional charge most times. I got an altered compression height (2.0 vs the 1.975 they offered) and a slightly smaller dish (22cc), Scotty Brown got it all done for me without additional cost.

 

The deck is not taller on those blocks. The pistons were the cause of the low compression.

 

Ford raised the deck heights on some of their blocks; this is a fact--Ford actually admits it. Best I can tell (I'm not a "Ford Guy") this was just a matter of re-setting the machining operation to remove less material from the raw casting; it wasn't like they needed a special taller casting to accomplish this.

I have not heard that Buick did the same, but I haven't checked them for myself. Wouldn't really surprise me to find that this happened.

As I've said in other posts, Chevrolet was feeling pressure because the engines in question weren't producing the advertised horsepower/efficiency. GM engineers pulled some engines from the warehouse at random to measure compression, and the compression ratio was lower than spec. Turns out the guy in charge of the engine machining operation was telling his people to cut the blocks to the tallest end of the spec. This saved machining time, and the tooling lasted longer before needing to be replaced/resharpened because less material was getting removed. He got bonuses for productivity by screwing the end customer and potentially violating the EPA fuel economy figures.

The engineers presented their findings, the main point being that the factory had proven they could hold far tighter tolerances than the manager claimed; which meant the allowable production variance could be more strict, which then meant that the blocks got made more accurately.

Moral of the story: "Spec" doesn't mean squat if the guy running the machine has been told to ignore quality for quantity.

 

It was mostly in the pistons. They actually have smaller dishes but the piston pin height was much lower. When you pull the pistons measure the pin height. The dish was something like 28cc instead of the 32cc from 71-74. If you just want a nice reasonably priced option just get a set of the "70" forged Speed Pro pistons +.030" which have a normal pin height, deck about .020" and depending on heads you'll be in the 9.75-10:1 range. Plenty of engines have made strong runners with that setup.

 

I know 100% that the 350 have a taller deck height in later years, and I agree it’s likely less machined off the block.

Jim would know if the 455 is the same or not, I’ve never measured a 455 so I’m not sure.

 

Why didn't GM "I'm not to sure of the Chrysler or Ford" just build the engines to the rated compression specs?
If a Nailhead is doing a true 9.5 to 1 or the 455 doing the same or a bit less I can't see a .75 difference meaning a true 10.25 having valve clearance issues? and both motors had options of 11 to 1 pistons on top of that, if it was even a true 11 to 1.
Pontiac iv'e read were no where near 10.75 either and i'm guessing it was across the board with GM back then.
I think Mopars were the closest to the legit rating with the highest hp stock 440 4bl. rated at 9.7 to 1

Was it detonation or a hotter running engine issue or a harder starting issue due to higher compression. Yes the gas back then had a options of over 100 octane but I know octane rating isn't the only thing for a good fuel.
I can see 25 hp. more from the rated specs?

 

Yep!
Been saying that for awhile, doesnt matter whos name is on the building, box, or product, if ONE person drops the ball, game over

 

Because most consumers are stupid and gullible. Horsepower was rated "at the brochure", not at the flywheel. Reducing compression with dirty tricks results in fewer warranty claims for holes in pistons and pounded bearings.

And they mostly got away with it.

Back then, gasoline was rated using a different measurement system. The fuel generally wasn't that much better than what's available now, with some notable exceptions.

 

Ah yes, the joy of having employees. When I had them, I knew that chest pain meant it was time to fire someone.

 

Production tolerances vs Engineering specs..

I am as sure as I can be, without having been there, that there was a process engineer or group that added a cushion to all the critical engineering specs, in areas where production could potentially be disrupted.. and pistons hanging out of the block would be game over for the line. So they added a .030 safety margin to the spec, and that was prioritized over any performance issue.

The engineering spec on the deck clearance is .010.

The way the vast majority of the customers drove, they would not notice the difference in performance, I am sure the complaints came in due to detonation, which they were more prone to do with the pistons that far down the hole. I recall talking to a couple techs at a Buick dealership I worked at briefly.. and they said that the 455 had a big issue with detonation complaints back then. Must have made a big impression, since it was 1994 when I was asking them... they recalled it distinctly. Especially on the 70 and earlier high compression motors, that were still in service in the mid to late 80's, when the fuel was reformulated for "clean air"..

The OEM's don't do anything without a reason, on the corporate level.. and since every BBB ever produced has the same generous specs, we must assume this was a corporate decision, not some guy trying to save time and tooling.

JW

 

Trying to keep clear of detonation was also one of the reasons for the mostly conservative initial timing and front end suspension rated advance springs in the distributor.

 

Thanks everyone, for all your help gonna tear into the bottom end this week, measure pin height and go from there. Try to give this thing a fighting chance

 

Couple weeks old, but when I tore my 75 block down, they were .090 in the hole, except for the two early style pistons somebody had put in it, which were about .050 down. It has a set of wiseco pistons in it now, pin height is 1.980 if I remember right, and they sit .045" below the uncut deck.

 

That's a shame. .045 is a lot to take off the block decks.

I'll never understand why piston manufacturers do that. They're professionals, they should know better.