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Old 05-17-2008, 04:27 PM
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Umbrella and viton seals on pre 1981 chevy small block?

Hello,
I am refurbushing the heads of a 305 chevy 1978 SB, and can't find a definitely yes or no answer on fitting my valve guides with post 81 umbrella and viton seals on it. Some people say it's a bad idea because the valves will lack lubrication, others say it's ok to do it and that lubrication will come through the positive seals and umbrellas, and that gas is a itself in charge of the lubrification job.
Who can help me doing the good choice?
Thank you

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Old 05-17-2008, 05:02 PM
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on all the small blocks we do valve jobs on they get positive seals, if the guides are clearanced right from the start, you wont have a problem. i have never seen any adverse problems from doing this.

sam-missle
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Old 05-17-2008, 05:18 PM
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Quote:
Originally Posted by bowtie6
Hello,
I am refurbushing the heads of a 305 chevy 1978 SB, and can't find a definitely yes or no answer on fitting my valve guides with post 81 umbrella and viton seals on it. Some people say it's a bad idea because the valves will lack lubrication, others say it's ok to do it and that lubrication will come through the positive seals and umbrellas, and that gas is a itself in charge of the lubrification job.
Who can help me doing the good choice?
Thank you
Lets start with gas as a lubricant, the answer is no. It cuts and cleans oils and greases as a solvent. In other words they go into solution with the solvents that gasoline is made of (acetone, toluene, etc) like sugar goes into solution with water.

Now seals, these and the material of the guide do have a relationship to oil. Cast iron guides would like more oil than bronze alloy guides. For a stock cast iron Chevy head, from the factory it ran the valve stem in a cast iron guide simply made by boring a hole thru a boss cast into the head. Refinishing the guide is done by knurling, making it bigger, or pressing in a new guide of iron or bronze alloy.

1) Knurling is a process similar to rolling threads, a tool is passed down the guide which rolls a spiral of material into the spaces of the tool. The guide is then refinished with a reamer to proper size for the valve stem to be used. This does three things to the guide:

- It forms a spiral path from top to bottom which holds oil but also provides an uninterrupted passage from the rocker box to the valve pocket. For intakes done like this, a positive seal works best as plenty of lube gets trapped in here and you don't want so much it runs thru with the suction of the intake system. For the exhaust an umbrella seal works well as the exhaust pressure tries to push the oil out back of the guide to the rocker box; so allowing more oil in is a good idea.

- It reduces the surface area the stem works against. This increases the wear rate which will result in more oil being drawn down the intake side guide in few miles so a positive seal will keep excessive oil consumption at bey for a longer time.

- It work hardens the rubbing surface, this will increase guide life if the reamer didn't completely remove the work hardened zone. It usually does to where this becomes a moot point.

2) The guide is resized to a standard oversize valve stem. This requires all new valves with the proper stem size. Operation and wear against the new cast iron surface and hard chrome valve stem is as original. Umbrella seals are sufficient, till the lift at the valve gets to about a half inch, then positive seals will hold the oil out longer as the guide wears and the clearance increases.

3) The original guide is bored oversize to accept a press in replacement. If the replacement is cast iron, the rules in ppg 2 work as the factory built it new. If they are a bronze alloy, much less lubrication is needed and both the intake and exhaust will be happy with positive seals. You can use an umbrella seal if you want as the stem to guide with bronze is usually a tighter clearance so oil going down the guide isn't such a problem as cast iron guides with their wider clearance. However, as valve lifts get to .5 inch and beyond, the side loads on the guide get pretty high and bronze will flow away from the load increasing the clearance and drawing more oil down the guide. As the lift at the valve starts getting around a half inch (12.7 mm) its time to consider positive seals.

Anyway them's my engine building rules and why I use 'em, others may have differing views.

Bogie
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Old 05-18-2008, 03:20 AM
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Thank you sam and old bolbie.
To be more accurate: in the head reconditionning jod I did, The machinist told me that my original guides were in good condition, my problems prior to disassemble the heads were that I had compression/fire leaks between some cylinders, the head gasket was 30 years old, plus I saw that I also had bad valves closures on all exhausts, and oily manifold intake ducts that did bad intake valve sealing too.
So the machinist did only a grind job on valve seats, plus the usual headgasket resurfacing job. I also put 16 new valves to complete the job.
So as reading your very sharp answers, I begin to think that It will be good to put umbrella on intakes, and Viton seals on exhauxts, keeping the O rings on top of stems at the same time. Is it right to do that?
Or should I leave the exhaust ones free of guide seals?
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Old 05-18-2008, 06:57 AM
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Here is my 2 cents based on my past experiences, limited as they may be.

0-ring seals.....pretty much worthless

Umbrella seals......ok until they get hard and brittle and fall off.

Positive seals....excellent seal for keeping the oil out of the cylinder, PROVIDING the bosses are machined to accept them. Not many older heads are.

Can you check this and post back what you find. With the new valves in the head, (no springs on yet), can you wiggle the top of the stem and feel any movement? any movement at all?

I ask because i'm finding it hard to believe that 30 year old heads did not need new guides.Putting new valves in loose guides, even with positive seals, rarely lasts long. The stem will wobble out the seal and the oil usage comes back.

Mark
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Old 05-18-2008, 09:02 AM
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Ok Jmark,
Here is what I found: with my 78 heads without springs, when dry, the exhaust stems show little movement, and the intakes ones, very little play noticable. If I wear a thin oil coat on them, there is no more movement on the stems. The use of new stems (standard size also did good in reducing that play).
The car had only 90000 miles, it was a flower hearse car before for years, and mechanicaly it seems have suffered from too many idling hours at low speeds on short trips. Carbon was present in the combustion chambers and on top of pistons.
The engine was previously performing well, my aim was to reset the engine in order to be sure of the internal condition of the engine.
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Old 05-25-2008, 03:50 PM
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OK, finally I decided to remove the vitons on exhausts guides, because even if the guides seem to accept the seals, I was not sure they will stay on, and that guides would receive sufficient oiling. I put the same umbrellas as on the exhaust, natural color with little rubber ring insert to stay on stems.
Hope it's the good choice.
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