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View Full Version : Locating valve guide in small engine



Robo
10-28-2012, 02:29 PM
I have a small engine head that has the exhaust guide "blown out" of it. The valve seat is in suprisingly good shape and should be able to be lapped back in with minor effort. Problem is I need to bore out the head and make a bushing to press the guide back into or use a larger diameter "repair guide". Is there a method of locating the center of the guide off of the "good valve seat"? I know typically its done the other way with piloted tooling but I don't have the piloted tooling or the guide centered to pilot the tooling. I put an indicator in my boring head and have it dialed in so so.....just wondering if I am missing something? Also the valves appear to be "square" in both directions to the gasket surface of the head. I currently have the head mounted on the table off of the valve cover machined surface (which appears to be parallel with the gasket surface) so the gasketed surface and valve seats are facing up. Any advice would be greatly appreciated

Toolguy
10-28-2012, 02:56 PM
I would get the gasket surface leveled out good with the indicator, then dial in the valve seat. Although it's on an angle you should still be able to find the center of it with a good indicator. Be as accurate as possible on the indicating and you will probably be good to go.

shawnspeed
10-28-2012, 07:13 PM
You will need to re-cut the seat after installing the new guide...it is the only way to be sure the seat is concentric to the valve head, in the new guide...it is not worth not cutting the seat, and dropping a valve because it was not centered, and or sealed correctly , causing the valve to fail....Just my 2C...Shawn

ormachine
10-28-2012, 10:39 PM
shawnspeed is correct. You will need to recut the seat !
I have been doing cylinder head work for 15 years and have never come across a replacement valve guide where you could get away without recutting the seat.

Ron

strokersix
10-29-2012, 08:27 AM
Make a tool like this: A round plug with 45 degree chamfer on one end and 11/32 hole thru. Do chamfer and hole in same setup. Place plug chamfer on seat and run 11/32 drill rod thru tool and guide. Drill rod long enough to stick up above plug. Indicate exposed drill rod both radially and axially. This will get you close.

Adjust size as required. 11/32 is inline and SBC Chevrolet valve stem size.

A.K. Boomer
10-29-2012, 11:15 AM
Robo - your going about it the same way I would - by all means indicate the seat first as this will get you closer to the center, beware on rocker type actuated valve trains the seats will be a little oval when indicating,

after your done a simple lapping might take care of it and if not a mild cutting then lapping,,,

but by all means indicate in multiple area's, what you don't want is to just take a wild stab at it and then have to cut the seat way off from center,,,

I would not build a bushing for the guide, that takes up too much clearance and you may either weaken the head or get into an oil or coolant or whatever port.
get an oversize guide or build an oversize guide...