Wimblowdriver · 2025-08-23T14:27:32+0100

DanClarke said:
I have the seals and I just need time to install them on all 8 valves.

On my classic triumph engines I like to put seals on the inlet valves only (none at all were fitted back in the day). The accepted theory being, this stops oil (and air) being sucked down the valve guide into the combustion chamber on the induction stroke. And not thought necessary on the exhaust valves, because there is never any vacuum between the exhaust valve guide and the combustion chamber. Also the exhaust valve stem needs good lubrication from the oil. The inlet valve stem gets a stream of cool air and fuel to keep it sweet.

marjon · 2025-08-23T14:31:53+0100

Wimblowdriver said:
On my classic triumph engines I like to put seals on the inlet valves only (none at all were fitted back in the day). The accepted theory being, this stops oil (and air) being sucked down the valve guide into the combustion chamber on the induction stroke. And not thought necessary on the exhaust valves, because there is never any vacuum between the exhaust valve guide and the combustion chamber. Also the exhaust valve stem needs good lubrication from the oil. The inlet valve stem gets a stream of cool air and fuel to keep it sweet.

Thats an interesting thought and I can agree (in my mind) is a sound theory.

J

DanClarke · 2025-08-23T15:19:37+0100

Wimblowdriver said:
On my classic triumph engines I like to put seals on the inlet valves only (none at all were fitted back in the day). The accepted theory being, this stops oil (and air) being sucked down the valve guide into the combustion chamber on the induction stroke. And not thought necessary on the exhaust valves, because there is never any vacuum between the exhaust valve guide and the combustion chamber. Also the exhaust valve stem needs good lubrication from the oil. The inlet valve stem gets a stream of cool air and fuel to keep it sweet.

I like the thinking behind this.
Mr. Google AI says the following....
For a typical BMC/A-Series engine rebuild, valve stem oil seals are often fitted to the inlet valves only, not all eight valves, as the exhaust valve side has positive pressure which helps to blow oil back into the rocker box. However, later A+ engines introduced a design with seals for all valves, or it is a common practice to fit them to all valves, especially if the valve guides are worn.

Why only inlet valves in older A-Series engines?

Pressure vs. Vacuum:
Inlet valves operate under a vacuum when open, which can draw oil down the valve stem into the combustion chamber. Exhaust valves, however, have positive pressure when open, which helps to prevent oil from entering the guide.
Material Requirements:
Exhaust valves run at higher temperatures, requiring specialized seal materials that were not always used in early A-Series engines.
Cost-Effectiveness:
Early BMC engines were designed to be cost-effective, and seals were not fitted to the exhaust valves unless absolutely necessary.

When to fit seals to exhaust valves:

Worn Valve Guides:
If the valve guides are worn, they create more clearance for oil to pass, regardless of the valve type.
Improved Performance:
Fitting seals to all valves is a common practice for a more complete seal and can reduce oil consumption and burning.
Later Engine Revisions:
Some later versions of the A-Series engine, such as the A+, came with seals fitted to all valves as standard.

In summary:

If you are working on an older A-Series engine, it may have originally only had seals on the inlet valves. However, fitting seals to all eight valves is a very common and beneficial modification, especially if oil consumption is an issue.

Wimblowdriver · 2025-08-23T16:06:51+0100

That's very informative. Does your engine have the grooves in the exhaust valve guides for the seals to fit? For my triumphs I had to get grooved inlet guide so that seals could be fitted. If you ever need new guides, look for those made from Colsibro, a high grade copper alloy, superior to bronze and iron types.

DanClarke said:
Worn Valve Guides:
If the valve guides are worn, they create more clearance for oil to pass, regardless of the valve type.

I would be wanting to replace the guides if they were so worn that they needed seals to stop oil passing.

Stanleysteamer · 2025-08-23T21:47:38+0100

DanClarke said:
They do say that every day is a learning day, and I think that is true.
Last night I watched the "Guru" for SU Carb tuning sort out another mini that was running very poorly.
The car in question was a 1275 GT Mini (a close cousin to my "Bitsa" Mini as far as engines go).
One of the takeaways from the clip was an item on plug colour varying across the 4 because of "insufficient coil power".
It was evidenced in the clip by the electrode porcelain of plugs 1 & 4 being "spotless" and plugs 2 & 3 being discoloured.

The actions he took were to reduce the plug gaps from 35 thou' (as he recommends with ultra high power coils and dizzys) down to 25 thou'. He then set the carburettor up to deliver correct fuelling and sent the owner out for a test drive.
Upon return he pulled one of the previously "dirty" plugs and it was now spotless just like plugs 1 & 4.
This really resonated with me, as I have 1 & 4 spotless and 2 & 3 being dirty.

So guess what? I reduced my plug gaps to 25 thou' and went out for the usual blast around here.
I came home and pulled #2 plug and it too is now spotless. Result!
Number 3 is also greatly improved but still a little bit dirty on one side of the porcelain, but this is for another reason (valve-stem seals) I think. I have the seals and I just need time to install them on all 8 valves.
But nevertheless I am quite pleased with this little improvement.

Glad you are pleased.
all 8 would be nice and neat, but you only need them on the inlets, basically.
If you've got 'em fit 'em!!
EDIT. Posted before reading all other opinions and other stuff.

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