So, I found the valve guides in my slant six are a bit too tall for the cam lift I plan to run. Time to take the head to a machine shop and get a bit of machine work done. Now the question I have: If I'm having the valve guides machined anyway, does it make sense to change them to run positive valve stem seals instead of the stock umbrella type? Pros and cons?
The 340 springs I'm running have plenty of room to clear stock umbrella type seals, so interference with the valve spring is not a limiting factor.
Yes, go positive seal. Preferably a Viton material.
Pros over an umbrella are: Higher quality material, less likely to create a "pumping" of oil into the guide, won't flop around and get cut/fail, controls oil much better, longer lasting, more clearance to spring.
Cons: extra costs but not much
On applications where more oil is preferred to get into the guide such as circle track (high rpm, or endurance) just remove the spring from the seal to increase the oil flow. In extreme cases I don't run a seal at all on the exhaust.
IMO the only reason to keep using umbrella seals is because you don't want to pull the head off and send it out to be machined. They "work" but this is 2020, we can do a lot better if you are willing to have the machine work done.
And yes, Viton. The cheesy white nylon seals belong nowhere near a functional engine.
One concern I've heard discussed is that some sorts of positive seals can get stuck to the valve stem instead of the valve guide, in which case they might as well be negative seals. How does one avoid this?
In reply to MadScientistMatt :
Only time I ever ran into a seal not holding the guide properly was a bad machine because of loose guide clearance (it wobbled around and caused the tool to cut small), trying to set the size with the incorrect tool, and lastly cheap/ or miss machined replacement guide.
In your case the machinist will have a STD fixed tool seal cutter. Your guides should be in good shape since its apart check them, if not, get the full valve job done.
I was planning on a full three angle valve job - especially since I may have nicked a seat or two porting the head. Now I just need to figure out what sort of positive seals would fit my valves.
In reply to MadScientistMatt :
Engine Machine shop should have them in bulk. Its all pretty standard sizing and minimum variations. Yours is likely 11/32 valve stem and if a .500 seal cutter is used that would be a US Seal VS-107 or VS-108. If a .530 seal cutter is used it'll be a VS-530. Different brands vary in part number and minimal OD size. As long as you ask for/make sure its a metal clad/body type like the VS-107 and VS-108 here: https://ussealparts.com/cylinder-head-parts/valve-stem-seals.html?guide_size=23&material=9&packs=497&stem_size=12&style=4&style_type=35
On my Datsun the motor sees between 4K-8K and is rarely below 5000 RPM, I used both types of seals and haven't noticed a difference either way. With that said I'd go with the positives seals if you're already having the work done.
FWIW I have never seen a viton type seal stick to a valve. It would have to be "wrong sized" for that to happen.
I had it happen on the Fiero from Hell. The wrong size seals were used.
The failure mode was a heavy oil leak that looked like typical Iron Duke valve cover leak if it weren't a new engine. Closer inspection revealed that the oil was leaking from the exhaust manifold gasket.
asphalt_gundam said:
In reply to MadScientistMatt :
Engine Machine shop should have them in bulk. Its all pretty standard sizing and minimum variations. Yours is likely 11/32 valve stem and if a .500 seal cutter is used that would be a US Seal VS-107 or VS-108. If a .530 seal cutter is used it'll be a VS-530. Different brands vary in part number and minimal OD size. As long as you ask for/make sure its a metal clad/body type like the VS-107 and VS-108 here: https://ussealparts.com/cylinder-head-parts/valve-stem-seals.html?guide_size=23&material=9&packs=497&stem_size=12&style=4&style_type=35
Thanks - I had been expecting to track down the parts myself, but if I can just bring the head and valves to the machine shop, and the seals are only a few dollars on their own, this sounds like it should be pretty easy.
