|The MGA With An Attitude
What to do when you encounter STICKY VALVES - CH-102
At 07:33 AM 3/23/99 -0500, Scott C. Williams wrote:
>.... unleaded gas .... sticky valves .... I think its those rotten bronze guides mostly, I'll never use them again. ....
Hey, quitcherbellyachin. It isn't the bronze guides, per se, but the installation.
I run hard valve seats, Stellite exhaust valves, bronze guides, and Perfect Circle valve seals, and I love the setup. The Stellite exhaust valves last at least twice as long as the stock valves, the hard seats run almost forever, and the guides have no noticeable wear when running Perfect Circle valve seals. Trust me, unleaded gas is not the cause of your problems, as these mods are the solution to the gas problem.
You are correct that the guides are causing your problem. The bronze guides expand faster with heat than the iron head, and the head is a solid block that's not going anywhere, so when the guides swell with heat they expand inwards and constrict on the valve stem. Because of this, the bronze guides need a little more running clearance than the original iron guides. Original clearance spec is 2 thou for the inlet valves and 2-1/2 thou for the exhaust valves, +/-1/2 thou. The bronze guides should be reamed out for about 3 thousandths of an inch minimum clearance.
A lot of shops will just press in the guides and install the valves, and if the valves move freely they finish assembly and send it out the door. I've had this happen to me, and when I brought it to the attention of the shop they quite effectively told me to take my business elsewhere, which I happily obliged. Since then, whenever I take a head in for valve work I disassemble it first and only take the head and valves to the shop. I leave the springs and retainers home so they can't reassemble it. Then when I get it home I can inspect the work before I put it back together.
My expedient fix for the sticky valve problem was quite simple, after I had to literally beat out one of the valves with a hammer and punch. I took an old valve, cut off the head, chucked up the bare valve stem in an electric drill, smeared on a little valve lapping compound, and ran it in the guide with the power drill to lap the guide until it would run very free with the lapping compound in the guides. This took about five minutes of lapping for the worst offender, a little less time for the other guides, less than a half hour total lapping time. Then when it was cleaned up and dry the new valves would drop right through by gravity, lift with your finger tip and they drop right back. Like magic, no more sticky valves.
For what it's worth, I only experienced the sticky valve problem right after the head rebuild, and only at 6000 RPM and extended full throttle while autocrossing (high heat conditions). I have never had one get sticky on me after it had been running well for some months. If the problem is now new after some months of running correctly, I would suspect some carbon buildup in the guides. This could happen if the valve seals are ineffective allowing oil down the guides, and then especially if you encounter an abnormally hot running condition, like boiling the coolant once, driving a rare high speed autocross event, or pulling a long hill up a mountain at speed. The immediate cure for the problem is still the same, open up the guide clearance a little.
Long term prevention is to fit good valve seals to keep oil out of the guides. Bronze guides are self lubricating with steel valve stems, no oil required, and still they don't wear out. After 100,000 miles you may find more wear on the steel valve stems than on the bronze guides. I just touched up my 100,000 mile cylinder head and transferred it to the next engine. All it needed was a little third angle touch up to narrow the lands a bit on the exhaust seats. The intake seats and bronze guides still look like the day they were installed.
So, it seems like you may need a head gasket set, a valve spring compressor, one old valve, some valve lapping compound, a power drill, and a new set of valve seals. When you get the springs off, measure the diameter of the top end of the valve guides. If your engine has double valve springs, the guides may have been turned down a bit to fit constricting type valve seals. I recently dug up part numbers and sources for a few different sizes of valve seals. Let me know what engine you have and what the guide top diameter is, and I can tell you what seals to buy.