This time it is a big G head.  Found out today one of the valve guides and bosses that centers the spring is cracked all the way down to the spring seat.  The machine shop believes and I understand the point that the guide and boss needs to be ground all the way to spring's seat.  He "thinks" a cupped type spring spacer will control the spring bottom.  Seems like the spring would walk around even with the cupped spacer.  Also, it seems like there wouldnt be enough guide to keep the valve from wobbling.  What do some more experienced think.  Hopefully this wont turn out to be another head going to scrap!  What a messy dissapointment.

I wonder, but dont know, if in the process of relieving the top of the guides a cutter didnt snag and produce the crack.  Valves were all removed when I took the heads in, and I had messed with spring height checking, but sure didnt see any crack.  It should have been impossible to miss, i.e. big. 

With this development, on top of the fact that the heads have been in "shop" for a full 6 months, there will be a new shop next goround.

Mike

Mike, located in the Siskiyou mountains, Southern, OR 292 powered 1946 Ford 1/2 ton, '62 Mercury Meteor, '55 Country Squire (parting out), '64 Falcon, '54 Ford 600 tractor.

Easiest fix is to completely replace the guide with a replacement stepped guide that bottoms out on the head from the top.  These guides are available in both iron or bronze.  This then gives a spot to hang a positive seal in place as well restoring the guide length in which to support the valve stem and help to stabilize the valve spring.

Lorena, Texas (South of Waco)

Thanks Ted, is there a preference, iron or bronze?

Mike, located in the Siskiyou mountains, Southern, OR 292 powered 1946 Ford 1/2 ton, '62 Mercury Meteor, '55 Country Squire (parting out), '64 Falcon, '54 Ford 600 tractor.

My own preference resides with bronze but be sure to run just a bit more valve to guide clearance with bronze than you do with iron.

Lorena, Texas (South of Waco)

Ted,

Try as I might (heads redone twice/two different shops ignored my specs), I STILL have only .002" exhaust guide clearance, and that is an optimistic measurement (hole gauge a VERY firm drag in the guide). I removed the little spring ring from the viton seals on the exhausts (assuming they are superfluous), and since the stainless valves need aggressive clean-up before installation anyway (oiled parts caught in a flood), what about chucking them in a drill and polishing the stems with fine scotchbright? Might reduce OD a tad and leave micro-scratches that would retain oil?

6 VOLTS/POS. GRD. NW INDIANA

P.S. Also curious re: my measuring technique. Since the guides/valves are not perfectly cylindrical/uniformly formed, what criteria is routinely used by machinists to calculate clearance? (there are high spots that range from 1 to 2 ten-thousands). Would the very small areas of sub-minimal clearance be a significant consideration if the vast majority of clearance is adequate? Do they average the #s, or use the high (or possibly low) readings to designate guide clearance?

6 VOLTS/POS. GRD. NW INDIANA

Unless you can do a ‘ground’ finish on the stems, sizing the guides themselves is the preferred method for maintaining stem to guide clearance.  You do not want any ‘scratches’ on the valve stems as that will simply speed up the wear process.  Although you can use a ball mic to check the guide hole dimension, a dial bore gauge is quicker and ultimately more accurate.

Lorena, Texas (South of Waco)

Isnt it great!  This shop has had the heads so long they forgot my instructions, which included:  Surface the G minimally to flatness, and then the 113 .012, plus the amount removed from the first.  The order was reversed, as the 113 was surfaced minimally.  When I found out I got back in there ASAP and told them not to do ANY surfacing on the G. 

Mike, located in the Siskiyou mountains, Southern, OR 292 powered 1946 Ford 1/2 ton, '62 Mercury Meteor, '55 Country Squire (parting out), '64 Falcon, '54 Ford 600 tractor.

[quote][b]Ted (5/17/2010) You do not want any ‘scratches’ on the valve stems as that will simply speed up the wear process.



Hmmm... Something to consider. "Speeding up the wear process" might be preferable to seizing an exhaust valve. If the stainless stems are indeed soft enough to be scratched by fine scotchbright, by the time the clearance has worn to proper specs, they might be polished smooth again, and the additional oil the scratches could hold until then might prevent a seizure in the meantime. As I plan to expend the effort to get geometry near perfect, total wear over time could actually even out. Will have to give this a good think, as there is no way I'm going back to either shop for more guide honing.

6 VOLTS/POS. GRD. NW INDIANA