# Piston Question



## Goat Roper (Oct 28, 2014)

What is going on here with these pistons?
These are out of a recently rebuilt 389 Pontiac engine.
There is a notch on the sides, should the notch face the front or the rear of the block?
This doesn't look like carbon to me but more of a baked on oil sludge.
Thoughts?


----------



## PontiacJim (Dec 29, 2012)

Goat Roper said:


> What is going on here with these pistons?
> These are out of a recently rebuilt 389 Pontiac engine.
> There is a notch on the sides, should the notch face the front or the rear of the block?
> This doesn't look like carbon to me but more of a baked on oil sludge.
> Thoughts?



Notch to the front. Pistons that are "left" and "right" typically have a means to identify which direction they need to be inserted, ie the piston has a rod offset or the valve notches have to be positioned to match the valves.


----------



## Goat Roper (Oct 28, 2014)

PontiacJim said:


> Notch to the front. Pistons that are "left" and "right" typically have a means to identify which direction they need to be inserted, ie the piston has a rod offset or the valve notches have to be positioned to match the valves.


So these pistons were installed backwards?


----------



## Shake-N-Bake (Jun 16, 2015)

Goat Roper said:


> So these pistons were installed backwards?


Looks that way to me. I would expect the notches to be towards the front of the cylinder bore. How were the rods installed? Is the chamfer on the big end away from the sister rod? If the chamfer on #1 & #2 are towards the outside of the crank journal then the pistons may be installed backwards on the rods. If the chamfer on the rods are in the middle of the journal then you need to take those out and flip them around. How tight is that bottom end? Does it spin easy?

Sent from my XT1585 using Tapatalk


----------



## Goat Roper (Oct 28, 2014)

Shake-N-Bake said:


> Looks that way to me. I would expect the notches to be towards the front of the cylinder bore. How were the rods installed? Is the chamfer on the big end away from the sister rod? If the chamfer on #1 & #2 are towards the outside of the crank journal then the pistons may be installed backwards on the rods. If the chamfer on the rods are in the middle of the journal then you need to take those out and flip them around. How tight is that bottom end? Does it spin easy?
> 
> Sent from my XT1585 using Tapatalk


The engine and pistons in the pics are not mine, it was another Central Virginia Machine Service nightmare that blew up.
This engine has now been torn down, destroyed parts replaced and re assembled by another builder.
Steve will be posting the particulars in this thread.


http://www.gtoforum.com/f170/centra...vice-cvms-negative-review-106018/index12.html

He is having trouble posting pics because of the file size so I am just trying to help another victim of CVMS out.


----------



## Shake-N-Bake (Jun 16, 2015)

That makes sense. I wouldn't expect an engine to last very long if assembled in such fashion. Most all pistons that I have ever used had a slight pin offset. I believe the purpose is to help balance out the rotational forces of the engine...(the piston is always being shoved against the cylinder walls in the direction of rotation). I suppose some special purpose Pistons may not have an offset pin but I am not familiar with those types. In regards to pin offset....there isn't a left or right bank set, all pistons are the same so it doesn't matter which bank they are installed just as long as they all are pointing forward (or backwards if you are building a counter rotation circle track engine).

The bank 1 vs bank 2 piston placement has to do with dome design. Domed tops or Pistons with valve reliefs may have a left and right set. Stock Pontiac Pistons have valve reliefs on both sides of the piston top so they can be installed in either bank....again, as long as they are facing forward.

When one attaches the connecting rods to the pistons, it's critical they are oriented correctly. Most connecting rods have a thrust (flat) side and chamfered side at the big end. The pistons for the forward bank needs to have the chamfer on the forward side of the piston. The other bank has the chamfer end on the rear side. Its a pretty easy mistake to make and I have seen it more times than I can recall. That is probably the very first thing I check when picking up my parts from a machine shop.


----------



## BearGFR (Aug 25, 2008)

:agree: Notch to the front, assuming that they were installed onto the rods correctly. Both orientations are important - the notches in the pistons (so that pin offset is correct) AND the chamfers on the rod bearing edges that are supposed to face towards the fillet at the 'end' of the crank rod journal.
It's possible to get them both wrong --- that is, getting the pistons installed on the rods 'upside down' so that when the bearings are right, the pistons are wrong (notches to the rear) ---- or vice versa: notches to the front, but bearing edges binding on the fillets at the ends of the rod journal.


----------



## geeteeohguy (Feb 2, 2008)

X3 on the engine being assembled with the Pistons in backwards. And looking at the pistons removed, it was pumping a lot of oil to do that....either due to the piston mis-install, or due to ring pack/cylinder wall finish issues. Can you say BLOW-BY? When I tore my '67 GTO down after 21 years of service and 173,000 miles, my pistons were MUCH cleaner than those. How is the new place in AZ??


----------



## Goat Roper (Oct 28, 2014)

New place is fantastic and I have to tell you I was sweating bullets transporting my firearms until I crossed border into America.
Shooting area is 5 minutes from my house and I was able to fire my AR-15 for the first time since '94.
The house is all set up, final move will be in the spring.
We are still dealing with DOBO and closing the business until the end of the year.


----------



## tiretread (Sep 28, 2015)

I am extremely happy I was led to this thread (and others) regarding CVMS. I have been saving my nickels and times to have my 389 rebuilt by Jim (or who I thought was Jim) in March. Fortunately, I was talking with a forum member over on PY that directed me to do a bit more searching on threads regarding CVMS. So very glad I found this but I am sorry for all the troubles you ran into.


----------



## Goat Roper (Oct 28, 2014)

Nothing I could do to recoup any of my loss but I sure as hell wasn't going to keep silent and let other Pontiac people get taken for the same ride.
I am so glad you found these posts and dodged a very expensive bullet.


----------

