loose piston clearance vs. street motor?

[kielbasa]

Hi guys, currently rebuilding my little 318 mopar...my brother is doing the block machining, he asked me if I wanted to run the pistons a little on the loose side....he mentioned somebody, sometime said something about marine engines having to run looser clearances on pistons and ring end gaps because of cold water always surrounding the engine, while the internals remain hot. Is there any truth to this???
FYI, just running some hypereutectics and regular moly coated rings.....

[Infomaniac]

Hi guys, currently rebuilding my little 318 mopar...my brother is doing the block machining, he asked me if I wanted to run the pistons a little on the loose side....he mentioned somebody, sometime said something about marine engines having to run looser clearances on pistons and ring end gaps because of cold water always surrounding the engine, while the internals remain hot. Is there any truth to this???
FYI, just running some hypereutectics and regular moly coated rings.....
I always add .002" to the manufacturers specs for piston/cyl clearance on marine engines.

[Moneypitt]

Hyper toon icks require an extra large top ring gap to survive, read the specs that come with the pistons. Failure to follow the specs WILL result in piston failure/meltdown..................MP

[Infomaniac]

Hyper toon icks require an extra large top ring gap to survive, read the specs that come with the pistons. Failure to follow the specs WILL result in piston failure/meltdown..................MP
I read his post too fast.
I do not ever run Hyper utec pistons.
Disregard my previous post.
:hammerhea :hammerhea :hammerhea

[GofastRacer]

Those Hyper pistons are ok for a daily driver and that's it, they are too brittle and they will break under heavy duty use!..

[steelcomp]

Those Hyper pistons are ok for a daily driver and that's it, they are too brittle and they will break under heavy duty use!..
Not only that, they'll fall apart under a heavy load. :wink: And they're not very durable, either. :)

[GofastRacer]

Not only that, they'll fall apart under a heavy load. :wink: And they're not very durable, either. :)
Exactamundo!.. :)

[kielbasa]

so are they worse than the cast slugs the factory put in????? this is just gonna be a 300 horse cruiser engine, now you guys got me scared.....my buddy ran hypers for years in his 340 mopar racing roundy round.....granted, it wasn't 600 hp, but he's never had a failure.....also, why would the type of piston require different top ring end gap?

[Jet City]

so are they worse than the cast slugs the factory put in????? this is just gonna be a 300 horse cruiser engine, now you guys got me scared.....my buddy ran hypers for years in his 340 mopar racing roundy round.....granted, it wasn't 600 hp, but he's never had a failure.....also, why would the type of piston require different top ring end gap?
As far as I understand the Hypereutectics ARE a cast piston, Hypereutectic (I believe) is aluminum that contains a silicone content greater than 9% (such as 380,390,etc.), Hypoeutectic would containe less than 9% Si (such as 6061,7075,etc.). I would guess these would have a higher silicone content than the factory cast pistons, probably means greater heat capacity and more brittle.
I helped my uncle select parts for a 383 SBC he built for his Hallett, he went with hypers (against my recomendation), its only been run a 1/4 of a season, so the jury's still out, I would choose forged if at all possible, the $200 savings could mean a pan full of gravel.

[Moneypitt]

If you read the info supplied with the pistons, the ring gap has to do with heat transfer to keep the piston shape equal, and round. I used them in a 383 mopar for my son's jet boat and so far, so good. The larger than "normal" ring gap was really stressed by Wild Bill who did the machine work.(BTW anyone know where he moved?) I've heard horror stories about the meltdown factor, but those stories didn't include this excessive gap in the top ring. Keith Black is no fool, these pistons went thru a lot of R&D prior to their release. They are rated far above the stock cast junk, and I'm sure a phone call to the Black shop would enlighten anyone thinking about using them. If you want the straight poop, go to the horses mouth...............MP

[RACER52]

we called them hyper-u-cracked it at the race track.
it takes a few blown engine's to get that reputation at the track.
my 2 cents

[Slyder]

If you keep the engine below 5500 RPM....and for short bursts....it should be fine. BUT...if you think you can beat the crap out of these things and they will LAST you will be sorely mistaken.
I had a 461 oval port engine in my V-drive with 10.2:1 Hypers. It ran great at the lake. The engine made great power and would run to 6500 with a 15 gear. With 18's I could run for a mile at 6000. Even went 10.52 @ 97 at Ming with that engine. THEN....I pulled the wrist pin out of the bottom of #6 piston....the steel rod spun around and sawed the block and the oil pan in half....tore up a cylinder head, crankshaft, camshaft and lifters, and left a pan full of aluminum gravel and a wristpin bushing!!
SO....learn from my mistake! They are great in very mild performance applications, but they will not take the abuse that a forged piston will endure.
Paul

[GofastRacer]

As I said before, they are good for a daily driver that's it!. A friend of mine used them in a hot street machine(never took it out to the drags)and lost a motor because of them, and btw that was without nitrous he didn't like the stuff!..

[steelcomp]

AFAIK...the main benefit with Hypers is that you can run a MUCH tighter piston to wall clearance because of the lack of expansion of this type of piston. This is due to the excess silicone. By running the tighter clearances your rings stay much more square in the bores so they seal better and last longer. I think most modern production engines are running Hypers as a stock item now. They are stronger than a typical aluminum cast piston, but will not take any kind of detonation, what so ever, nor will they take a lot of on and off, on and off, hard throttle action. Those guys getting away with using them in circle track motors are doing just that...getting away with it. The extra wide top ring gap is because they retain so much more heat then a std piston...again, because of the high silicone content. More heat transfered to the ring, more ring growth, more gap needed.
If you're not going to run the tight clearances required for these pistons, you're really not taking advantage of their true design intention. There are even high silicone content forged pistons on the market that are doing the same thing...trying to control the expansion of the piston so you can run tighter clearances, again, keeping the rings more square in the bores for better sealing.
Kielbasa...to answer your original question..yes, it is common practice to run the bore a little bigger for a boat motor with an open cooling system due to exactly what you said...hot piston, cool motor syndrome. I've run BBChev's with as much as .012" clearance, depending on the piston.
steel

[LakesOnly]

Those Hyper pistons are ok for a daily driver and that's it, they are too brittle and they will break under heavy duty use!..
Indeed, hypers in a sustained hi-rpm dealio like our boats often end up as gravel at the botttom of your oil pan (and in your galleries)
LO

[Squirtcha?]

Besides all that..........it's way too hard to spell, or pronounce hypereutetic (or whatever it is).

[Floored]

having worked at Keith Black's when Keith was alive I can tell you he would'nt have his name on those POS pistons. Family is just making money off of a good mans name and tarnishing his rep. Its a shame.

[steelcomp]

having worked at Keith Black's when Keith was alive I can tell you he would'nt have his name on those POS pistons. Family is just making money off of a good mans name and tarnishing his rep. Its a shame.
KB's rep is still all green with me!! :D

[Sleek-Jet]

Besides all that..........it's way too hard to spell, or pronounce hypereutetic (or whatever it is).
Quite possibly the best argument for forged pistons I've ever heard. :D