low compression in one piston

[ptokun]

i have a question if any one could help me? a while back i over heated my motor really bad i melted my piston ring to my piston and broke my valve so i rebuilt my motor changed pistons and valves and valve seals, changed my head gasket got arp head studs and put it back together i did a compression test today and that piston that melted read only 75psi the rest were 130 130 and 150 thats still pretty low. im not sure what the problem is im guessing its my cylinder, should i get it bored out or just honed and should i change my rings again? im new at rebuilding motors, if any one could help please

 

[TSi Kid]

I would do a leak down test before ripping into your block again.

 

[bryanwheat]

Did you not hone the cylinders? Also did you lap the valves when you installed them?

 

[mrbill86]

Whenever you rebuilt the head, did you have it resurfaced? If you got it that hot, you most likely warped the head which will cause low compression. Also, what the other guys said.

 

[ptokun]

i never honed my cylinders, im wondering if thats all i need to do or do i need to bore it also, and im not sure what lap your valves means. i didnt get my head resurfaced i just cleaned the surface with a razor also the block surface before installing the new head gasket

 

[bryanwheat]

To get the new rings to seat properly the cylinders have to be honed. It is hard to tell if they need to be bored without seeing a picture of it. Also you should of had it checked for straightness after getting it that hot. It is likely warped. Lapping valves is when you use a lapping compound (grease with grit in it) between the valve and seat. It will remove any imperfections and show you what kind of contact you have between the 2.

Valve Lapping - YouTube

 

[ptokun]

thanks ill get my cylinders honed and head resurfaced. after i do this do you think its necessary for me to change my head gasket and rings again i only got about 15 miles on it?

 

[92AWDHX40]

Well if your pulling the head, change the gasket.

 

[Alex7747]

thanks ill get my cylinders honed and head resurfaced. after i do this do you think its necessary for me to change my head gasket and rings again i only got about 15 miles on it?

A head gasket is only 30$, why wouldn't you? And if i were you, i would completely tare it down and make sure everything is in spec before you put it back together again. You don't want to rebuild it for a third time do you?

 

[bryanwheat]

I would replace the rings no questions asked.

 

[Hitmachine101]

thanks ill get my cylinders honed and head resurfaced. after i do this do you think its necessary for me to change my head gasket and rings again i only got about 15 miles on it?

yes you must put a new head gasket back on again, it sucks but once you torque that head down, you have to replace the head gaskt and head studs ( assuming there stock felpro head studs ) arps are different, ive heard you can re use those i think

and yes get the head resurfaced, new valve guides if you didnt the first time, and new cam seals ( do it right so you dont have the rip is apart the third time ) as well as laping the valves, as the guy above posted how to do it

just went throught this process and im glad i did everything at once, cost me 250 from a machine shop, worth the piece of mind

 

[bryanwheat]

yes you must put a new head gasket back on again, it sucks but once you torque that head down, you have to replace the head gaskt and head studs ( assuming there stock felpro head studs ) arps are different, ive heard you can re use those i think

and yes get the head resurfaced, new valve guides if you didnt the first time, and new cam seals ( do it right so you dont have the rip is apart the third time ) as well as laping the valves, as the guy above posted how to do it

just went throught this process and im glad i did everything at once, cost me 250 from a machine shop, worth the piece of mind

Why replace the guides unless they are cracked or out of spec.

 

[92AWDHX40]

Hg, rings why not your already there, and whatever you may have missed before, check the head out, hone the cylinder walls, i would even have the block decked to make sure its flat. I dont like surprises so i tear things down thats just me, doing this so long i learned from experience. Do it once and, have some peace of mind.