I have read the previous threads on this subject but i couldn't find one that covered my exact experience.
I've had a 1974 F100 with a 360ci for a couple of years and recently it developed a puff of blue smoke on start up, it does not smoke when running on the road and it drives great.
Eventually the truck had real problems starting, the only way round this was to remove all spark plugs and clean, then it fired up. As this became an issue i replaced the valve stem seals with Felpro parts and the heads in place, i didn't see any obvious issues with the rockers, valve springs or valve stem play in the guides.

With everything back together yesterday i fired the truck up and all looked good, i left it idling until it got to temperature and went to drive off when i noticed huge plumes of blue smoke from the right hand bank exhaust pipe. I did some minor fiddling but nothing serious as i ran out of time.

Does anyone have any idea what i might have done when i replaced the valve stem seals and where to start looking ?

The original valve stem seals were quite badly worn with oversize bores
The new Felpro seals fitted snugly on the valve stems
I used compressed air to hold the valves up
I put assembly lube on both ends of the push rods and valve tops
The R/H rocker cover has the PCV valve connected to the rear of the carb
I made sure the thinned down rocker shaft bolt went into the correct thread

What is confusing me is the engine never smoked before carrying out this work but i can't think of anything i may have done.

Where should i start ?

 

Maybe you got one of them on wrong or torn.

 

It's very common on FE's for the oil returns in the head to gunk up and hold oil back around the valves. this increases oil burn, if you didn't check them you need to.

If your guides are worn bad enough new seals won't help much, the clearance is simply going to allow too much oil down the stem.

You could use a heavier oil and add something for oil burning such as Rislone or similar products. they really do help in older engines you're trying to get a little more out of.

 

It is possible that i may have torn one but i was extremely careful and did each individual valve in turn

 

I did wonder about a possible blocked or partially blocked oil way, are there just 2 in each head, one in each lower corner or more ?

Interesting comment about a heavier oil or additive

 

Oil viscosity and additives will help. anything to slow the amount of oil getting down the guide and into the cylinder. . the better additives will also help soften seals and help with leaks if you have any.

I'm a sceptic and never buy into snake oils, but I've had enough success with some additives over the years I know they can help, depending on the cause of course they aren't magic.

 

Pull the valve cover and visually inspect all the seals are correctly in place and return holes open. If everything looks good leave the cover on loose, start engine, after a couple minutes or when it starts smoking carefully remove cover and see if oil is backing up in the head as 440 said. It is possible some pieces of the old seals broke off and blocked the return holes.

 

I took the rocker cover off and checked the oil returns on the smoking bank and one was blocked with quite large and hard pieces of old valve stem seal backed up with a fair bit of sluge. I decided to remove the rocker shaft so i could check and clean everything just to be sure.

Funny thing is the stem seals i took off were all complete so these pieces must have been from a previous seal change.

So, thankfully my smoking issue has now gone away, it's surprising how quite a major running issue can be such a simple fix

Thanks for all your help, it certainly gave me the right direction to start looking and i'll know to check these oil returns in the future

I really appreciate everyones help with this