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Valve guide replacement on 1948 Pontiac Silver Streak


Teddy
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I have a 1948 Pontiac Silver Streak 6cyl flathead. It had been in storage for many years and I just want to get it running. I took the head off along with the manifolds, and I removed all of the valves since some were stuck. The cylinders look good and the valve seats are good. I bought new valves, and was thinking of just lapping and installing. I was not planning to replace the valve guides because I assumed that it would be too difficult to do with the engine in the car. But the ID of the exhaust guides are significantly eroded near the top although reasonably good further down. How hard is it to replace the guides, if possible at all, with the the engine in the car?  Since I have not seen a new guide, not clear whether they go in from the top or bottom. But based on another post with good information about fabricating a drift tool, it appeared they must be driven out from the top. Is there is a shoulder on one end to seat the new guide at the proper point when installed. If the the new ones have to be driven or pressed in from the bottom, that appears to be a difficult situation because of the limited working room. And I also saw in the shop manual that the new guides are undersized and are supposed to be reamed with a special tool. Any comments would be appreciated.

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If they aren't too badly worn they can be knurled back to size. This is easier and cheaper than replacement and will last quite a while. If you are not doing a full rebuild it could be the best way to go.

 

You probably don't need to replace the valves unless they are badly worn, burned or corroded. An old time auto machine shop could grind the valves and seats and knurl the guides if necessary, with the engine still in the car

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Thanks for the reply. The intake valves and guides were probably in good enough condition to reuse, but many of the exhaust valves were worn enough that I wanted to replace since I had the opportunity. And the portion of the exhaust valve guide ID near the top that is eroded is probably beyond the point that knurling would help. My hope was that maybe the guides could be replaced with the engine still in the car, but not familiar enough with it on an engine like this, and did not want to try to remove the guides and then find that installing the new ones was not easily possible.

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I do not know the answer to your question as I am not familiar with valves in a flat head engine, but I know enough to get me in trouble with flat head engines, BUT @Bloomight be best to not only know the answer but get you in the correct direction, and for any Pontiac OEM parts would be @Kornkurt

 

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Thanks for the reply Bloo. I was able to find some information online about the valve guide design, which is a straight tube, with no shoulder, and is driven out from the top, and the new one installed from the top. The depth that it is driven in has to just be measured to match the original. But the issue with doing it with the engine in the car is that the guides can be very tight and require drilling the ID out to make it easier to remove and to avoid damaging the block. With the limited working space, particularly for the back cylinders, and the concern that they may not come out easily, a local mechanic that I talked to advised that it was high risk to try to do it in the car. So I may just lap the new valves and put it back together for now and wait for another opportunity to replace the guides if I take out the engine at some point in the future.

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If there is little side play when you wiggle the valve in the guide ( with the spring off) then you will be good for a while, even if the top is corroded. By little I mean less than maybe .020 -.025 at the head. (yes, that is a lot to a machinist).  just saying what you can "get away with"  the looser it is, the shorter will be it's life. 

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