What kind of thread is this???

[Dadandson]

I need to know the thread of stock valve adjusting screws on a 74 1275 OEM rocker arm. It appears to be a 9/32-26 British Standard Fine Thread. All of my handbooks show this thread is obsolete! Can anyone help? Could it be a M7 X 1.0? Thanks

 

[Trevor Jessie]

Wow! I know the answer, but can not pull it from my brain. The answer came about as we were discussing the click-adjust tool.

Maybe it will spring back into my head in a minute.

 

[Trevor Jessie]

OK, I made a call. Apparently, last time we discussed this we never found a proper answer and the person used a "work around".

I doubt it is a metric thread. Are you looking for a tap?

 

[Dadandson]

Yes. I can purchase a tap, if it is metric, but I would like to confirm thread size first. The .275" major diameter does work to 6.98MM and there is only one thread pitch, 1.0, for 7MM thread. I am engineering and manufacturing, what I hope to be, affordable roller rocker arms for A series motors. I am very close right now. The key to my product would be a "No machine" one for one swap with the stamped rocker arms. No extra parts to buy unless the OEM parts are worn. They even fit under a stock valve cover!

 

[budk1953]

Can't confirm the thread, but if it is BSF this may be the tap...
Taps

Bud

 

[Trevor Jessie]

Well... right there it is (as you said) BSF 9/32-16.

 

[Trevor Jessie]

BTW, if you are making custom rockers, what lift ratio are you "aiming" for?

 

[Dadandson]

My goal is to increase the ratio, maybe 1.2 or 1.3:1, but not to point of having to machine the head for push rod clearance. Remember, I want to install these rocker arms without removing the head. I think that would be the appeal of these.

 

[Trevor Jessie]

Can't you get almost 1.3 with offset bushings?

 

[Dadandson]

Offset Bushings? Do you mean an out of concentric bushing in the rocker for the rocker shaft? Sounds like some machining involved.

 

[Trevor Jessie]

https://www.aptfast.com/Images_Parts/Cams_Valve_Train/A_Parts/OSRB-A.jpg

BTW, I think the later adjustment screws were 5/16 UNF.

 

[Trevor Jessie]

No machining required, but they would need to be precisely placed when pressed into place.

 

[Dadandson]

I have never seen these before. Question: I have maintained the stock dimension from the center of the rocker shaft to the center or contact point of the roller on the valve. I have found that moving the adjusting screw .100" closer to the fulcrum increases the valve opening by .046". Do these bushings move the rocker .100"?

 

[Trevor Jessie]

I dunno how much they move the rocker. Call David at APT and ask him. Or contact Hap, he might know.