bluethunder73
New member
I have a 75' 392e motor and had custom pistons made and can't remember which valve the valve relief is for. This is kinda important for installing the pistons. Any help is apprieciated.
392 valve relief
- Thread starter bluethunder73
- Start date
Robert Kenney
Super Moderator
The factory 392e has a d shaped notch in the piston but it is not a valve relief but a decompression notch. See attached picture on the up notation in the middle and the notch. The rounded side ov the d goes down. If you have a camera take a picture for us to id youe deal.
bluethunder73
New member
Thank you
Robert Kenney
Super Moderator
In case your pistons have a positive dome, the reliefs May in fact be for the valves easy to tell which is for which valve is the intake valve stem is perpendicular to the deck and the relief will be machined straight into the piston and the exhaust valve is about 15 degrees to the deck and the relief would be cut at an angle similar to the d in my picture. I attached a cylinder head picture for reference. Big valve is the intake
bluethunder73
New member
These are actually flattop pistons for the emissions model 392 so if I took your first post right I installed them with the d-cup down. I cant get a picture now but if you were standing on the rear of the motor and looking at cylinder one I put the cup on the outside of the motor.(closer to the exhaust than to the intake valley.
Robert Kenney
Super Moderator
Yes, the d is down and on the out side of the motor, same side as the exhaust valve.
Michael Mayben
IHPA Tech Moderator - Retired & No Longer Online
Good information here! I'm gonna move this thread to engine tech sub-forum which is a more appropriate location.
Curious about the wiseco pistons...are the piston pin bosses not "offset" as is normal practice? And if so, were the pistons not marked in some manner regarding orientation to the rods for proper installation both on the rods and in the block?
The piston top markings would be something similar to the oem piston I.d. With an arrow or the word "up", or maybe a dimple (or any combination?).
I don't know what the oem piston pin offset is, that spec is not referred to. But I'd suspect in the range of 0.040">0.080"???? Maybe Robert has actually measured this on an oem piston for verification?
I do know that "piston slap" is very prevalent in the 196 motor (same dimensions as a 392), no doubt it's a mixture of a possible oem piston material selection combined with not optimum piston pin offset due to rod angularity. Robert is the math dude here and knows much more about the rod angle deal regarding various bore/stroke relationships when dealing with stroker motors than me!
I'm also assuming based upon your other thread, that this block is also a m/y '75 source so it must be improved cooling?
Curious about the wiseco pistons...are the piston pin bosses not "offset" as is normal practice? And if so, were the pistons not marked in some manner regarding orientation to the rods for proper installation both on the rods and in the block?
The piston top markings would be something similar to the oem piston I.d. With an arrow or the word "up", or maybe a dimple (or any combination?).
I don't know what the oem piston pin offset is, that spec is not referred to. But I'd suspect in the range of 0.040">0.080"???? Maybe Robert has actually measured this on an oem piston for verification?
I do know that "piston slap" is very prevalent in the 196 motor (same dimensions as a 392), no doubt it's a mixture of a possible oem piston material selection combined with not optimum piston pin offset due to rod angularity. Robert is the math dude here and knows much more about the rod angle deal regarding various bore/stroke relationships when dealing with stroker motors than me!
I'm also assuming based upon your other thread, that this block is also a m/y '75 source so it must be improved cooling?
Robert Kenney
Super Moderator
When I drew up a custom 392 piston for a special application I did not see any pin offset. I have since not measured any on a 345/304 piston either. Doesn't preclude pin offset on a sv piston just have not run into one yet....
Pin off set from what I have learned and read was used to optimize rod angle on the power stroke and also to minimize piston slap on long stroke / short piston applications. Some racing teams (nascar??) utilize it to optimize ve when tuning for best power on restricted class racing. GM uses it fairly often for these reasons. May be other reasons but I don't know.
Pin off set from what I have learned and read was used to optimize rod angle on the power stroke and also to minimize piston slap on long stroke / short piston applications. Some racing teams (nascar??) utilize it to optimize ve when tuning for best power on restricted class racing. GM uses it fairly often for these reasons. May be other reasons but I don't know.
bluethunder73
New member
I am not for sure but remember the mention of an offset on the pistons. And I will have to get back to you on the improved cooling since the block is at my school. And me plus three others couldn't find a marking as for which direction to face the piston. And thank you r. Kenny your information was very helpful. And to seal the deal I will check for sure when I check for proper valve to piston clearance.
Michael Mayben
IHPA Tech Moderator - Retired & No Longer Online
This is really a kool project/motor build! And it's certainly out of the ordinary!
One of our lurkers here has drug in his wieand-blown puller 392 motor out of the weeds for rejuvenatin'. And since he's a machinist/fabricator/blacksmith, he can do some awful nice one-off kinda schnizz so I know he'd be interested in your project also!
I've delt with thousands of wiseco pistons over the years, but never in automotive apps, all my experience with the stuff is for both two-stroke and four-stroke race engine apps in pro-level motorcycle engine building. For some of that stuff, wiseco was the only choice!
The reason for asking about the improved cooling deal, I just had to! Robert has just completed a motor build where the owner did not pay enough attention to that detail and ended up with a highdollar set of heads that did not match the block!
I do know from much experience in dealing with race motors built by others...it's very easy to install some pistons "backwards". That means if there is piston pin offset designed in, if installed backwards, catastrophic failure results at extremely high rpm, not to mention a terrible "clanking/knocking sound at certain rpm!
It's critical that the sv pistons are installed in the correct orientation on sv rods, due to the lubrication paths and thrust directions for the big ends. Since you are dealing with total custom rods and pistons, we're learning new stuff here and we need to know the big picture!
I had no idea that wiseco would do custom stuff like this nowadays...that brings up a whole other scenario for the stroker 152 that Robert and I have been plackin' on for the last year! In texian..."plackin'" means..."play likin'"!
Are you documenting this build with pics? I'd really be interested in the rod/crank/block clearances once the short block is assembled to further our knowledge about this stuff. And...this particular bore/stroke combination just might be the relational hot ticket!
One of our lurkers here has drug in his wieand-blown puller 392 motor out of the weeds for rejuvenatin'. And since he's a machinist/fabricator/blacksmith, he can do some awful nice one-off kinda schnizz so I know he'd be interested in your project also!
I've delt with thousands of wiseco pistons over the years, but never in automotive apps, all my experience with the stuff is for both two-stroke and four-stroke race engine apps in pro-level motorcycle engine building. For some of that stuff, wiseco was the only choice!
The reason for asking about the improved cooling deal, I just had to! Robert has just completed a motor build where the owner did not pay enough attention to that detail and ended up with a highdollar set of heads that did not match the block!
I do know from much experience in dealing with race motors built by others...it's very easy to install some pistons "backwards". That means if there is piston pin offset designed in, if installed backwards, catastrophic failure results at extremely high rpm, not to mention a terrible "clanking/knocking sound at certain rpm!
It's critical that the sv pistons are installed in the correct orientation on sv rods, due to the lubrication paths and thrust directions for the big ends. Since you are dealing with total custom rods and pistons, we're learning new stuff here and we need to know the big picture!
I had no idea that wiseco would do custom stuff like this nowadays...that brings up a whole other scenario for the stroker 152 that Robert and I have been plackin' on for the last year! In texian..."plackin'" means..."play likin'"!
Are you documenting this build with pics? I'd really be interested in the rod/crank/block clearances once the short block is assembled to further our knowledge about this stuff. And...this particular bore/stroke combination just might be the relational hot ticket!
Dealbreaker74
Member
Smokey yunick was the first person I ever heard mention "piston pin offset." in the days before custom pistons were available, he would install them "backwards". This created a condition in the combustion stroke where when the crank is at it's tdc, the rod (because of the pin being offset in the piston) had already broken the plane, and gone past its vertical position. This makes it much easier to push the piston down the bore when the crank is already fighting the force of combustion which began 30 degrees or so earlier. It was an effective way to minimize detonation, and makes more power in a race engine, though at the expense of greater "piston slap."
he was a real American character. Pick up his autobiography. A great read (but not for the kids!).
he was a real American character. Pick up his autobiography. A great read (but not for the kids!).