Cylinder Head Drain Question

raleypc · Monday at 8:43 AM

My 75 Commando will typically blow out a dense cloud of smoke on startup which will last for a moment or two. It then runs great with zero smoke after it clears out. It is not wet sumping as I have drained the sump (a few tablespoons of oil) and it will still smoke. Even tried an anti sump valve (since removed). I recently pulled the rocker covers and found a pretty deep pool of oil around the intake valve springs. The exhausts were well drained. I was able to run a piece of safety wire down the drain hole through the cylinder head and cylinder. The level MAY have dropped a bit there is still maybe 1/2" of oil remaining in the cavity. Does the oil drain down the pushrod tubes when it is running? Does there need to be positive pressure inside the cylinder head to draw the oil out? I suppose intake valve seals would solve the problem but I hate to pull the head as there are zero leaks and otherwise the engine runs great. I never had this problem with any of my previous Nortons. Any advice is appreciated!

kommando · Monday at 8:53 AM

The drains are the pushrod tubes and the single small drain on the right side, all are gravity so when the engine is running oil can be moving up and down. The single drain is vertical through the head, barrel and the very short first section in the timing side crankcase, it then takes a 90 degree turn and I have found a blockage there and had to probe it to clean it out. Blowing some air in from the top would tell you if yours is blocked. Intake seals were factory fit so should be present but could have failed.

raleypc · Monday at 9:10 AM

kommando said:
The drains are the pushrod tubes and the single small drain on the right side, all are gravity so when the engine is running oil can be moving up and down. The single drain is vertical through the head, barrel and the very short first section in the timing side crankcase, it then takes a 90 degree turn and I have found a blockage there and had to probe it to clean it out. Blowing some air in from the top would tell you if yours is blocked. Intake seals were factory fit so should be present but could have failed.

Thank you! I will see if I can blow some air through the small drain. Did you have to probe from the timing cover side to clear the blockage at the 90 deg turn?

kommando · Monday at 9:18 AM

I had the engine apart, so did it from both sides, assembled then you can only access from the top.

cliffa · Monday at 5:51 PM

raleypc said:
My 75 Commando will typically blow out a dense cloud of smoke on startup which will last for a moment or two. It then runs great with zero smoke after it clears out. It is not wet sumping as I have drained the sump (a few tablespoons of oil) and it will still smoke. Even tried an anti sump valve (since removed). I recently pulled the rocker covers and found a pretty deep pool of oil around the intake valve springs. The exhausts were well drained. I was able to run a piece of safety wire down the drain hole through the cylinder head and cylinder. The level MAY have dropped a bit there is still maybe 1/2" of oil remaining in the cavity. Does the oil drain down the pushrod tubes when it is running? Does there need to be positive pressure inside the cylinder head to draw the oil out? I suppose intake valve seals would solve the problem but I hate to pull the head as there are zero leaks and otherwise the engine runs great. I never had this problem with any of my previous Nortons. Any advice is appreciated!

FYI, you don’t need to pull the head to replace the stem seals….

Post in thread 'Replacing Valve Stem Seals (in situ) Pitfalls?'

Apr 26, 2022

There are a few older threads on this procedure. Ludwig's write up and tool cobbling guide for in situ seal replacement:

I used his homemade compressor but went with the "rope trick" method to hold the valves up, not his sparkplug hole-holder.

acadian · 2026-08-25T04:31:02-0600

BSA's use small orings in this application, always wondered why the same couldn't be applied to a Commando, just need to open up the hole in the gasket

Tornado · 2026-08-25T10:23:28-0600

I cleared the inlet rocker box drain using a long spray can type straw (think it was off a SeaFoam can) to probe well into the drillings, then apply some good sprays of wd40. Seemed to help speed up drainage of a measured oil pour in at the top. ...just time it before and after.
Ultimately the smoke out one side upon throttle blips on my 850 was found to be from failed oil seals on the valve stems...using the in situ method above, both just crumbled apart while removing. I used KibbleWhite Viton replacements...been great for 15k+ miles now.

ilikevelos · 2026-08-25T18:13:23-0600

Viton seals rock. I'm a corvair guy and viton pushrod seals transformed the oil tightness of corvairs.

Torontonian · 2026-08-25T18:46:42-0600

Your intake valve seals are now more than 50 years old .
I use the J.S. Motorsports type now .

ilikevelos said:
Viton seals rock. I'm a corvair guy and viton pushrod seals transformed the oil tightness of corvairs.

Never use standard rubber intake seals . Viton .

Cylinder Head Drain Question

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