Broken valve guides

[Roger G.]

Hello all,

Im looking at a set of 3931063 427 heads that are for sale locally. A couple of the valve guides are broken. Is this an expensive fix? Not sure how they got broken, should I be looking at anything else? Thanks, Roger

[Domenic T.]

Re: Broken valve guides

Roger,
I remember a similar problem on a SB head and the valve guide was machined out and a new one pressed in.
I also had a set of SB heads that had all the guides replaced with bronze/silicone guides.

DOM

[Duke W.]

Re: Broken valve guides

I'm not sure what you mean by "broken", but I assume you are talking about the valve guide boss than protrudes above the floor of the rocker box. (A photo would help.) Any set of used heads should be disassembled and thoroughly inspected for valve guide wear, seat and valve condition, etc., unless they are accompanied by documentation of a recent thorough rebuild.

There are a number of valve guide renewal techniques, so you need to visit local machine shops that offer valve guide rebuilding, but before you do, check the following Web site to get an idea of what is available. Not all machine shops will offer every method that is available to the industry.

http://klineind.com/k-line-service-t...m/guide-liners

Duke

[Edward J.]

Re: Broken valve guides

Roger, broken guides maybe from the valve keepers or retainers hitting the top of the guide?? generally a higher than normal lift camshaft with high rev.s, Maybe Duke can comment if this is the case.

[Roger G.]

Re: Broken valve guides

Thanks for the replies. Yes 2 of the bosses that protrude are partially broken off. These heads have been magnafluxed and shotpeened, are in good shape other than the guides, and the date codes are spot on for my engine. . I assume the guides can be pressed out by the machinist and replaced, but wanted to get educated before going down that road.

I did a search on here before posting, and two issues seemed to be mentioned. First was that these can sometimes leak when replaced, and the second was use of iron vs brass guides. These heads are fairly easy to find, and I can keep looking, but its a good deal. Also I wasn't sure how often the guides have to be replaced anyway on a 46 year old set of heads being rebuilt. Any comments appreciated.

thanks, Roger

[Domenic T.]

Re: Broken valve guides

Roger,
took a look at the web site and those are NOT for broken guides nor is the run of the mill shop. Head rebuilders may be your answer. There were companies that actually welded heads but most local shops only do the easy work. One tool does all 3 angles.
Sounds like your time is worth more than looking for the expert shop that can do these.
Now as far as cast iron, one rebuilder said that cast was better than any other insert? I do aircraft inserts and they are brass. Again, they have been brass since day one in aircraft as the industry is years behind. A good machine shop would know what to do, but the expense?

DOM

[Roger G.]

Re: Broken valve guides

Roger,
took a look at the web site and those are NOT for broken guides nor is the run of the mill shop. Head rebuilders may be your answer. There were companies that actually welded heads but most local shops only do the easy work. One tool does all 3 angles.
Sounds like your time is worth more than looking for the expert shop that can do these.
Now as far as cast iron, one rebuilder said that cast was better than any other insert? I do aircraft inserts and they are brass. Again, they have been brass since day one in aircraft as the industry is years behind. A good machine shop would know what to do, but the expense?

DOM

Thanks Dom,

This is the thread on cast iron vs bronze https://www.forums.ncrs.org/showthre...t=Valve+guides
Not sure that the question was answered though. Actually, there are a couple of local shops that can do this type of thing, but they are closed on the weekend, so I haven't been able to call them, but I also wanted to be able to ask the right questions before calling. Like you said, the expense?😊

Roger

[Duke W.]

Re: Broken valve guides

Thanks for the replies. Yes 2 of the bosses that protrude are partially broken off. These heads have been magnafluxed and shotpeened, are in good shape other than the guides, and the date codes are spot on for my engine. . I assume the guides can be pressed out by the machinist and replaced, but wanted to get educated before going down that road.

I did a search on here before posting, and two issues seemed to be mentioned. First was that these can sometimes leak when replaced, and the second was use of iron vs brass guides. These heads are fairly easy to find, and I can keep looking, but its a good deal. Also I wasn't sure how often the guides have to be replaced anyway on a 46 year old set of heads being rebuilt. Any comments appreciated.

thanks, Roger

All OE Chevrolet cast iron heads have what are called "integral valve guides". The guide surface is precision machined/honed right in the cast iron. There is no insert. Since you said the heads have been Magnaflux inspected I assume they are cast iron because Magnaflux only works on ferrous materials.

Aluminum heads have inserts because most cast aluminum alloys are not tough enough to resist rapid wear.

Typical inserts to renew cast iron head valve guides will not work because they don't include a boss. Guides for aluminum heads usually do. I believe your heads are reparable, but you need to consult with a machine shop with experience repairing this type of damage to understand the full range of options and the associated cost.

Duke

[Roger G.]

Re: Broken valve guides

Thanks Duke, I did not know they were integral to the head. Sound like this "good deal" is turning into an expensive one


Roger

[Dan P.]

Re: Broken valve guides

All of the Big Block heads that i have had had removable guides.During normal rebuild you would have to replace some of them any way . MAKE SURE who ever does the work knows which way they drive out . If they drive them out the wrong direction they will crack the head . I worked as a Chevrolet truck mechanic in 70's and worn valve guides was a common problem.

[Joe L.]

Re: Broken valve guides

Thanks Duke, I did not know they were integral to the head. Sound like this "good deal" is turning into an expensive one


Roger

Roger----


All small block heads, except aluminum, use the integral valve guides. However, big block heads have removable guides. The guides are removed by driving them out from the combustion chamber side and driving them in from the top side. The big problem with replacing valve guides in this way is maintaining concentricity (i.e. so that the valve head will seat perfectly when the valve is installed in the new guide). However, a competent head rebuilding shop should have "been down this road" a few times.

Actually, though, depending upon how bad the broken guides are, it might no be a really big problem, anyway. If you use umbrella style valve seals, as were originally used on most 65-69 Corvette big blocks, the broken guides might not be a problem (depending, of course, on just how bad they are which I cannot tell without a photo of them).

I've really never seen big block valve guides break providing the valve guides were not machined for positive-type valve seals. However, when the guides are machined down for positive seals, the remaining guide material can be fairly thin and can break. I can't tell if these have been machined, though, without a photo.

[Roger G.]

Re: Broken valve guides

Hello Joe,

Sorry no pics. i walked on them, and actually have found a nicer set that is cheaper

Roger

[Duke W.]

Re: Broken valve guides

Roger----


However, big block heads have removable guides. The guides are removed by driving them out from the combustion chamber side and driving them in from the top side. The big problem with replacing valve guides in this way is maintaining concentricity (i.e. so that the valve head will seat perfectly when the valve is installed in the new guide). However, a competent head rebuilding shop should have "been down this road" a few times.

I never knew that... never worked on big block heads, just small block heads. I checked the P & A catalog and only found valve guides listed in Gr. 0.283 for the early six-cylinder engine, so I assume Chevrolet didn't consider them replaceable.

What kind of material are they, and can they be replaced or just sleeved with bronze inserts? Do you have any photos of the guides, either OE or aftermarket?

About 15 years ago I had to have the guides replaced in my Cosworth Vega cylinder head. Fortunately a CV owner owns a machine shop in San Clemente and changing valve guides in aluminum heads is routine for him. He placed the head in an oven and heated it to about 400F, then removed it from the over, tapped out the old guides and installed the new, which had been chilled to below freezing temperature in a refrigerator. Then the head was placed back in the shut off oven and allowed to slowly cool over the next couple of hours.

I had hand selected guides (made by SI Valves in Simi Valley) to get exact clearance of about 1-1.5 thou, and hoped that the valves would then slide in without reaming, but Bob didn't think that would work. Sure enough, with the head back down to room temperature the valves were all a little sticky, so the guides had to be reamed. As the head shrinks during cooling the guides get ever so slightly distorted and need to be reamed. The valve seats and valve seating surfaces were in good shape, so I didn't want to grind them, which would have required reshimming to get the proper clearance, so I just lapped the hell out of them and got nice concentric seats.

If the valves need to be ground, it's always done after the new guides are installed and reamed, so the refreshed seat is concentric with the new valve guide.

Aluminum has about three times the thermal expansion rate of cast iron, so the above method works well, but might be tougher with iron heads. I believe that modern HD diesel engines have nickel-bronze valve guides in iron heads, and replacing them is probably considered SOP for major engine overhauls.

So, what's the normal field repair procedure for big block valve guides other than reaming the guides oversize and replacing all the valves with oversize stem versions?

Duke

[Joe L.]

Re: Broken valve guides

I never knew that... never worked on big block heads, just small block heads. I checked the P & A catalog and only found valve guides listed in Gr. 0.283 for the early six-cylinder engine, so I assume Chevrolet didn't consider them replaceable.

What kind of material are they, and can they be replaced or just sleeved with bronze inserts? Do you have any photos of the guides, either OE or aftermarket?

About 15 years ago I had to have the guides replaced in my Cosworth Vega cylinder head. Fortunately a CV owner owns a machine shop in San Clemente and changing valve guides in aluminum heads is routine for him. He placed the head in an oven and heated it to about 400F, then removed it from the over, tapped out the old guides and installed the new, which had been chilled to below freezing temperature in a refrigerator. Then the head was placed back in the shut off oven and allowed to slowly cool over the next couple of hours.

I had hand selected guides (made by SI Valves in Simi Valley) to get exact clearance of about 1-1.5 thou, and hoped that the valves would then slide in without reaming, but Bob didn't think that would work. Sure enough, with the head back down to room temperature the valves were all a little sticky, so the guides had to be reamed. As the head shrinks during cooling the guides get ever so slightly distorted and need to be reamed. The valve seats and valve seating surfaces were in good shape, so I didn't want to grind them, which would have required reshimming to get the proper clearance, so I just lapped the hell out of them and got nice concentric seats.

If the valves need to be ground, it's always done after the new guides are installed and reamed, so the refreshed seat is concentric with the new valve guide.

Aluminum has about three times the thermal expansion rate of cast iron, so the above method works well, but might be tougher with iron heads. I believe that modern HD diesel engines have nickel-bronze valve guides in iron heads, and replacing them is probably considered SOP for major engine overhauls.

So, what's the normal field repair procedure for big block valve guides other than reaming the guides oversize and replacing all the valves with oversize stem versions?

Duke

Duke-----

The stock big block valve guides are either cast iron or steel. I'm not sure which. I cannot find that these were ever available from GM in SERVICE. Most of the time folks use thin wall brass inserts for guide replacement. Thick wall cast iron or silicon bronze are also available. All of the aforementioned install by drilling the existing guides over-size and installing the new guides into the existing guides. Complete new guides are available in either cast iron/steel or silicon bronze.

Here is a photo of a replacement big block valve guide. This one is a Sealed Power VG 689. It's hardened cast iron. This is NOT a repair INSERT. It is a complete new guide.


bigblockvalve guide.jpg

[Duke W.]

Re: Broken valve guides

Thanks for posting the photo. Now the next question is: What is the replacement method? Is the head heated up in an oven? What temperature?

It's easy to replace guides on aluminum heads since the aluminum expands more with temperature than the iron or bronze guides, but if both the head and guide are iron, high temperature may not loosen them up much, so I can imagine that a lot of old guides break trying to extract them and installing new iron guides may result in similar problems.

A shop that specializes in HD diesel engine heads would probably be the best way to go since they should be set-up and experienced replacing valve guides in cast iron heads.

Also as you noted in your email to me, the exhaust guides are "wet", which raises the possibility of leaks, so an experinenced shop is a must.

This reminds me of my Titanic research over the years. Given that the overlapping hull plates of that era had riveted lap joints, I always wondered if some kind of sealer was used, especially considering that the mill finish was not that smooth and the plates would pickup some corrosion sitting in the yard prior to installtion.

Apparently not. If the red hot rivets were installed and peened properly, the plates were pulled together so tight as the rivets cooled and shrunk that leaks were rare. The Titanic had over 3 million rivets, and though not all of them needed to be water tight, a lot did, which made for a lot of potential leak points.

After the rivets cooled, checkers came along and tapped them them with a hammer. The sound told the checker if the rivet was properly installed and tight - a real art and probably a lost one at this point in time.

Duke