oversized valve stems and guides

[Joe Mish (43421)]

Is there a preference for machining and installing new valve guides vs simply using a new valve with an oversize stem on a smallblock street motor?

Seemed that years ago valves with oversize stems were used rather than installing replaceable guides and standard sized stems. Observed also that only the affected valve would get the work vs today's all or nothing at all philosophy.

Will a street engine respond negatively in terms of longevity, idle and street performance with a couple of heavier valves?

Thanks,
Joe

[Duke Williams (22045)]

Re: oversized valve stems and guides

There's nothing wrong with using oversized stem valves. IIRC the GM replacement oversize stems were .003", but I don't think they are available any longer for vintage Corvette engines, and I don't know if the aftermarket offers them either. The slight additional mass will have no signficant impact on valve train performance. Also, guide wear must be less than .003" so a .003" oversized reamer will form a new straight and round wear surface.

A number of valve guide rebuild tecnologies have been developed in the last 50 years, and they appear to be more popular and should restore all guides to proper clearance while allowing the old valves to be reused if stem wear and seat margin are within acceptable limits.

My criterion for reusing valves is no more than .0005" stem wear. Often inlet valves can be reused, but exhaust stem wear is usually greater than on theinlet vavles, so from a cost standpoint, valve guide rebuild may be cheaper than buying a new valve.

An advantage of using an oversize stem valve is that the guide could be reamed by the dealer mechanic and a new valve installed, so the head didn't have to make a trip to the machine shop and the customer got his car back quicker.

You might think of the oversize stem valve as a "repair" rather than a "rebuilt".

Duke

[Verle Randolph (14764)]

Re: oversized valve stems and guides

The aftermarket does have +.015 stems for the old cars, cannot find +.003 any more unless someone has some leftovers.

I would use oversize stems if possible and install guides only if there was no other choice. It may be remote but there is always the possibility of a guide slipping and causing catastrophic damage.

Verle

[Joe Mish (43421)]

Re: oversized valve stems and guides

Duke, Verle,

Thanks for your response.

Joe