5.0 small block -> sluggish pistons ...

[Rob73-87]

Hi everybody,

this time I want to tell you about a problem I had with my new (bored 0.30 over) 305cui motor.
I have collected a bunch of 305cui engines over the past 2 years - nobody did want them - they were cheap and I thought, maybe one day I will be glad to have them in the garage, - who knows ?

Whatever I decided to get one bored over, polished the crank and I brought the stock rods and new speed pro flattop pistons to my nearest machine shop (I habe been there a couple times earlier) and had them installing the new pistons on the old rods (press - fit !!!)
Well, everything went fine at first, but as I got the rods back, I noticed that 2 of them were quite sluggish, - and one piston was so tight on the rod I nearly couldn`t tilt it....
I wanted them to do those 3 pistons again, but they disagreed and told me to put them in - it`ll work ("the pistons will spin themselves free" ???)

Okay - so I installed the engine with the bad feeling, knowing that something might be not "right".

THe result was,- as I tried to break it in, it made a noise and stopped immediately, everytime I varied the rpm a little. Same problem whenn it was hot and I started it again...NOISE and then it was running....hmmm

I decided to take the engine out again, disassembled it, just to find out - those 3 pistons rubbed a little on top of their cylinders, just then, when the pistons stopps and begins to go down in its cylinder...

So WHAT DO YOU THINK ??

I will never again use pressfit pistons, - I will get fullfloating pistons for my 383 engine - I just didn`t want to spend money on new rods for this 5.0.....

[N2TRUX]

Sounds like you need to have a talk with your machine shop.

[chevybuldr]

When a piston gets tight like that it means that it is galled in the piston pin hole. Pistons should be put on rods with heat. A rod heater is what most shops use. Dont want the rod glowing red or it will loose its press. Now there are times when a pin needs to be moved a little and a press works for this. The pin needs to be well into the piston on both sides. Sounds like your shop pressed the pins causing the rod to give a high spot on the piston. Now I talk about this from experiance. When I have a piston that gets tight I take it apart and hone the high spot out of pin. The only thing that I dont understand is you say it is rubbing on top? Can you show pictures please.

[Rob73-87]

Originally Posted by chevybuldr

When a piston gets tight like that it means that it is galled in the piston pin hole. Pistons should be put on rods with heat. A rod heater is what most shops use. Dont want the rod glowing red or it will loose its press. Now there are times when a pin needs to be moved a little and a press works for this. The pin needs to be well into the piston on both sides. Sounds like your shop pressed the pins causing the rod to give a high spot on the piston. Now I talk about this from experiance. When I have a piston that gets tight I take it apart and hone the high spot out of pin. The only thing that I dont understand is you say it is rubbing on top? Can you show pictures please.

I do not have any pix right now - but I will take some tomorrow - I would appreciate it very much if you take a look at it

Rob