Exhaust Valve Guide Clearance
 

What should the exhaust valve guide clearance for BBC 502?

Valve guide I.D. - valve stem thikness = ?

Normally aspirated or supercharged? Is this a stock 502 or something that will be leaned on?

Stock HP 500 with 250 blower duel holley 850's

I am pretty sure the number I heard was .0017 - .0020 or thereabouts for a performance marine application. FWIW, AFR's site says their guide clearance generally runs .0013 - .0016, but they recommend checking with your engine builder for heavy nitrous usage, blower applications, or marine endurance applications.

Would that spec be different if there was no blower just N.A. carb?

I believe you can go a little tighter on a NA application, because there should be less heat generated in the chamber. You need to have a wider valve seat on a performance marine engine too, for heat transfer. Of course, if you don't have the tune right and lean it out and send the combustion temps through the roof, you could still have problems in spite of having the guide clearance and the valve seat on the money.

I would advise talking to one of the pros on the board, particularly someone who does a lot of head work. Eddie Young, Bob Madera, Haxby, MER - all of these guys contribute regularly on OSO, and all of them are very willing to answer questions. I am not a pro engine builder so I cannot offer much in the way of specifics, only what I have gleaned talking to some of these guys. Nothing like getting the story from the horse's mouth, so they say.

.0018

I set all of our marine heads at .002 on the exhaust guide clearance with 11/32 valves. If the guide is straight with no taper you will have no problems. We also run the blue viton seal on int and ex side. This clearance works on almost all applications the only time we alter from this is if we are going to run into very high egt's like on a turbo charged engine. Then we will run steel exhaust guides with bronze liners so the bronze guide will not chip and run threw the turbos. We check all the heads that come threw the shop even the new ones because 90% of the time they are on the tight side. Joe Irwin

Let me see if I have this correct.

Valve = 11/32" (.343) + .002 = .345. So the guide clearance is .345?

What you do is measure the valve stem diameter if its .3410 then you make the guide .3430 which will give you .0020 clearance. Valve stem diameter varies from manufacturer so you have to measure the valve that your going to use. Joe Irwin