MTFCA Forum

I'm replacing my two-piece valves (with .326" stems). The engine runs nicely, and I notice it has aluminum pistons. My question: how do I know if I should get oversize stems? Also, since I'm replacing the valves, should I also replace the old valve springs while I'm at it?

Phil

Springs are cheap buy new ones.

Do your self a favor. Research Model T valve cup failure.
I only put new valves from 351/390 in the engines I build. Using modern locks and keepers.

Low rpm low compression engines can get away with worn valve stems or guides, for awhile. One unavoidable problem with running worn stems or guides, is that wobbling valves wear the seats quickly, and result in block seats being badly damaged. In the days before replacement hard seats, that often resulted in blocks being scrapped.
The main reason to replace old "two piece" valves is to avoid the valve head from breaking off the stem while the engine is running. Sometimes engines might survive such a failure, often the block or head is destroyed suddenly. Two piece valves breaking was a serious problem on old model T engines in the early days of our hobby, and each year as the engines and valves continue to age, the failure becomes more likely.
Proper fitting of replacement valves of course is a wise decision, as proper fitting stems result in a better running and more reliable engine.

Here is what I am talking about.
https://www.mtfca.com/discus/messages/5 ... 1450555390
Don’t know if the problem has been fixed, but don’t care. Still use modern valves when I do engines for people.

To check your guides, you can buy gage pins at mcmaster-carr for about $6.00 in .0005 increments. Buy the low and high of the tolerance based on the actual replacement stem and total stem clearance spec. The vendor may be willing to mic the stems for you.

I had a valve cup fail on an engine that I just put together and when I took it apart there were multiple had cracks where the little pin goes through i’m almost every one. So I created a solution. I made a set out of billet stock. I wanted to make sure that never happened again because that’s a good way to get stranded,. Thank God, I was able to find the little pin. So that taught me not go cheap and try to reuse old parts unnecessarily

Thanks for the replies. I may need to go with the .030 oversize stems (.340" diam.) as my .326" stems do have some wobble. What should the ideal valve guide fit be? .002" over the stem diameter?

It was not used parts I am talking about. It is the reproduction washers and pins.
That is why I only install modern valves and locks in engines I do.
When have you heard about a 390 dropping a valve cause the lock failed?

Dan,

Can you post a picture of the modern valves and locks you use?

Tim

If there is so much wear that you need to use oversize stems, then you also most likely need to ream your valve guides oversize too. It's not only the valve stems that wear. When the guides wear, they almost certainly do not wear uniformly over their entire length. Just using larger stems will reduce excessive clearance but will not necessarily align the valve properly with the seat. Apologies if you already knew this and had already planned to ream the guides & seats to suit your new valves.

(Can you get away with doing none of this? Maybe. Model T's are very forgiving. But if things don't go well, at least you might know the reasons why.)

Same ones Langs sells.

I measured the valve guides at .331 - .332 in diameter using a hole gauge. They are definitely not perfectly even in I.D. I plan to install the .030 oversize valve stems and ream the guides with the valve guide reamer from Snyder's.

Phil

.0025-.003" is the total clearance. In that case you would feel some wobble. If you find a gage pin that is a slip fit to the guide, mark it up with a sharpie pen and slide it straight down the bore. Look for an even ink wear all around.

I wouldn't go any bigger on the stem size than you have to, to clean it up !