Piston questons

[John Fitzpatrick]

I’ve got two questions concerning my pistons. I pulled the heads on my engine to measure the chamber volume and figure the CR (the engine hasn’t been run yet) and noticed there is a conical indention in the piston centers, about ¼” in dia. I am assuming these were put there when the pistons were turned on a lathe to get the proper compression height, but that’s a guess. Will they pose a problem in use—perhaps cause a hot spot and detonation? Second, the pistons are 0.007” above the deck at TDC—is that any problem?
Engine’s a 289 0.030 over block, Boss 302 rods and crank, forged aluminum pistons, 1970 351W 64cc heads with 2.02/1.6” valves and a 10.2:1 CR.
Thanks

 

[Jac Mac]

Most Speed-Pro & TRW forgings have that center locator, sand down any sharp edges with some wet & dry paper if concerned.

.007" + protrusion with 0.038" gasket will give 0.030" piston to head. Thats at the bottom end of the 'acceptable limits' and will depend on your 'cold' piston to bore clearance and the amount of piston rock at TDC.

Jac Mac

 

[Rick]

Without being negative, is the corrosion on the head surface something to have looked at? It could present a sealing problem for you cooling system.

 

[Jac Mac]

John,
I hope my eyes & parallax error are playing tricks on me, but are those 'new' valve clearance notches cut correctly. The furthest away should be the exhaust & smaller in dia but deeper as a rule. To check , remove the valves from one chamber- bring piston to TDC- set head in place without gasket-grind center point on old valve stem & mark piston top by pushing pointed valve stem from top- remove head- use dividers on each center mark to check concentricity & radial clearance for each valve size.

Jac Mac

 

[Dimi Terleckyj]

Hi John

As Rick mentioned I have a concern with the amount of corrosion pitting that appears on the block and on the head.
If that is as bad as it appears it would be in your best interests to have it machined before re assembling as it will not provide a good long lasting seal if the head gaskets are seated on a poor surface.

I may be wrong as it is difficult to see in the above pictures.

Dimi

 

[Doc Watson]

To me it looks like the indentation is a result of using a lathe to turn the OD of the pistons, to stabalise the metal when cutting a tail stop is used which keeps the cylinder turning accuratly on the lathe. In order for it to locate on the metal being worked on a small recess is cut into the center of the metal to be worked on, cut at the same angle as the tail stock quill. I assume pistons without this are turned and then the top is machined to remove the hole after.

Back in the day I could turn a piece of metal to within 1 thou using only a pair of calipers and a steel ruler. It was also fun when someone set the tool post feed to a screw thread setting and turned on the lathe at high speed, the tool post would race towards the chuck and scare the #### out of you. Then there was the welding steel toecapped boots to vices....while someone was wearing them........ lol

Andy

 

[Malcolm]

I am definately not an engine man and my eyes are not always 20/20, but could that surface pitting actually be the top surface of the gasket peeled off and left behind on the metal of the head/block? The clue to make me think this is the blue lines showing which clearly come from a gasket. Unless it is a blue printed engine!!!!!!!! Then again it could be pitting......

 

[John Fitzpatrick]

The metal surfaces aren't corroded- what looks like corrosion in the photo is residue from the head gaskets which I hadn't gotten around to cleaning off, and some of the artifacts in the piston photo are bits of corn starch that I dusted them with to release the clay used to measure the pocket volumes.

Jac Mac-
Will do- thanks for the heads-up!

BTW there's an outfit on the other side of town that will evaluate your heads on a flow bench for $50, so I think I'll see what these vintage things will flow. I'll post results if anyone's interested.
John