Hello all,

I am baffled with this one. First off this is about a 350 boat engine with 0.040" over SRP forged pistons, 5.7" rods with a 3.75" forged crank. Static compression is at 8.6:1 and I am running a prochargers (600b) at 6 psi with a water intercooler. I had a head gasket fail between pistons 3 & 5 so I tore down the engine. Everything looked fine and measured within spec except the pistons. I measured 1/2" up from the bottom skirt and found all the pistons at 3.033, about 0.005" too small.

I have two boating seasons on the engine (about 30-40 hours). The boat is mainly used for water skiiing/wake boarding so it lives most of its life at part throttle. Full throttle runs don't last more than a minute. I've called SRP and the tech person just wrote this off as piston collapse and responded like this was a daily occurrence. Can anyone shed some light here?

The tune was good (EFI), and zero sign of detonation.

Any input on this?

Thats news to me, how the heck can a piston collapse?!?
I'm just tagging on to this thread to learn.

.007" clearance on a cold forged piston. You think it should only have .002" there? I thought forged pistons run wider wall clearances because they grow more, but I was guessing .005" or so?

Also, good god, you need a blown 383 motor to pull friggen water skiers? How the heck fast are you pulling them?! Yanking sumo wrestlers up to 60mph or something?

@ Sonix.
I would think this would be a normal measurment for a non-hyper forged piston. I'd get the real measurment spec from SRP before jumping to conclusions. I was amazed how much taper the forged TRWs I used had.
I'd bet money that they're supposed to be measured at the bottom of the pistons.

SRP pistons should be measured 1/2" an inch from the bottom. The minimum clearance should be 0.002" to 0.0025", but this is typically only for naturally aspirated street applications. An application such as this would be set up with more clearance. That additional clearance would be created by going a hair larger on the bore. It would probably be a good idea to ask SRP for a recommendation for the clearance for your specific application. I would expect the recommended clearance to be fairly large on a forced induction boat application. Even though the piston measurements don't seem right this application may very well need a 0.005" to 0.006" minimum clearance.

I have to admit that hearing of an SRP piston collapse is a new one to me. To have all eight collapse the same amount just doesn't sound right.

Are you certain your measurements were accurate? Have you verified your tools to make sure that your instruments are not off by a hair making your pistons look smaller than they really are?

What are the bore measurements? Are they all dead on 4.040" or are they slightly larger? If they are 4.040" on the nose I would believe that either the engine wasn't set up right in the first place or that the pistons were machined a bit on the small side.

How do the skirts look? Do they still look new all the way across or are there signs of wear towards the center of each skirt?

It would be hard to imagine the pistons changing shape without any issues, but perhaps if the clearance was too tight and the pistons were squeezed by cold cylinder walls and reshaped slightly while they were hot. I just can't imagine it happening without cracking the pistons, scuffing, or flat out corking the pistons in the bores.