reseat the valves aftedr new guides?

boggla500 said:

Ok I have a pontaic 455 heads 6x the machine shop says I need to reseat the valves with new guides I never heard of this but I don't know every thing and wondering how true is this or are they trying to take advantage. My machanic mentor says it bs. The bill to due this is enough to buy used bare pontaic aluminum heads soooo Iam not realy buying it. Oh and its Car Quest. any help would be great!

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The valve seats are machined with reference to the guide. Typical guide repairs don't take into consideration the location nor angle the original guide had with the seat. The typical guide repair starts with the guide in what-ever shape its in and either:
1) reams it oversize for a larger stem valve

2) reams it oversize and presses a tubular guide into the hole to restore the original valve stem size.

3) runs a knurler thru the original guide to expand the material, then a reamer is passed thru to size the hole for the valve stem.

You will note that all of these processes never use anything other than the original guide hole to locate the rebuild or repair of the guide. Since the tools used are not guided in 3 dimensional space, especially with small, fast and dirty shops. So these set ups take the path of least resistance thru a worn guide, thus there is no way to assure the guide is located either in the center of the existing seat nor aligned to the seat at a 90 degree angle. This requires that the seats be re-shot to insure that they are concentric and at right angles to the guide.

Bogie

I get this all the time, especially from the kids that buy those"bargain" camel back Chevies for $300.00 then find out the forty plus year old heads are crap.

Replacement guide work is usually done from the original OEM guide as previously stated. Reality sets in when you find that heat, wear, unleaded gas and all the other things that can happen to engine parts does it best to the guides. On a well used engine the guide wear will physically move the centerline of the valve seat. Even if the guide were replaced on the original centerline. The worn seat would not be correct.

Then you get into the case of a Big Block Chevy for example. That has replaceable guides as manufactured. If you replace these in the shop you will find the seats aren't even close. This is because the factory uses semi-finished guides and finishes them with a "gang" reamer setup that does all the guides at once. The bores are never in the center of the guide. All replacement guides use centered bores.

ANY guide replacement work= Seat work..... I pray for you if you use lapping compound on modern engines!!