NH 350 Valve Guide Removal

[MikeHenstock]

Many thanks for the responses, the job is bigger than I thought. I'll weigh up putting it back together with the wear to the existing guide against finding a engineer to install new ones for me. Thanks again.

[john.whiting]

The secret to removing guides from an iron head is to use a sufficiently heavy hammer.....around 3 to 4 lbs is good......Light hammers simply burr up the guide ,break small pieces out,and are other wise hopeless.......one or two well struck blows will remove any guide........they are only a light interference fit,maybe 0005.....however ,if the guide is driven through the port,then carbon ,ash and rust must be removed with a sharp scraper down to bare metal......if this isnt done ,the guide may well score the head bore ,and make a new guide a loose fit.......if the guide is driven from the spring side ,any collar needs to be removed......Simplest way to do this is to run a drillbit down the guide just far enough for the collar to fall off.

[john.whiting]

You dont need a lathe to check guide ID to OD concentricity.......just spin the new guide on the new valve stem.....you can easily detect as little as a few thou eccentricity.......A loose guide /worn valve stem will make a cockeyed seating in the head ,and need a cut anyway.......regardless of the accuracy of the new guide......Most guides (even in India) are made on CNC machines ,and are accurate straight from the lathe ,and are then centreless ground for an exact diameter.

[volker.halbach]

See the thread "valves and guides for NH" I started two weeks ago. Do not assume that all guides and valves are ok. I have new guides that give an interference fit of 0.004" in my cylinder head. Often the materials are not compatible and valves get stuck.

[john.whiting]

A guide 004 oversize on the OD has likely missed the grinding stage of manufacture.,this being a normal grinding allowance........if you look on some of the Chinese sale sites ,you can see massive lots of automotive components that have been rejected, for sale for fractions of a cent each in lots of 10,000.or 100,000 or more.,and this is where the junk engine parts sold by some of the importers come from.

[nevhunter]

Centerless grinding doesn't guarantee the hole is centered requiring removal of too much metal from the head to get a good seat. This "pockets" the valves and mucks up the rocker geometry making the guides wear faster. Nev

[john.whiting]

if we assume the guide is cut from a bar in one drilling ,reaming,OD turning,end form and part off operation,then the hole should be exactly concentric with the OD....To get the extreme runouts reported ,the guide would have to be drilled in a separate operation,likely on a manual drillpress.......With the lowest standard of Indian or Chinese cottage industry manufacture ,anything is possible.

[nevhunter]

Even drilling it in a collet without a steady against the drill can give a lot of run out. Boring it is the best and do the outside surface at the same setting if you don't have a fixed size reamer. Nev

[Paul Gasson]

With respect to all who have commented, I manufactured many turned parts as an engineer and we would have machined a part like a valve guide on one machine as John suggested BUT where concentricity was critical we would have bored the hole after drilling undersize or possibly as a secondary operation ground the OD on a mandrel between centres. A small diameter drilled hole can wander considerably in a deep hole as the bit is unsupported. Centreless grinding did not give the necessary concentricity and took no account of drill wandering.