Recommendations for how to check Piston to Valve Clearance

[Brett James-McCall]

Two questions really.

1/ Anyway got drawings to make up your own TDC stop

2/ What is the length of the lifter I have to turn up on the lathe, assuming I am running a standard hydraulic lifter

Thanks
Brett

 

[Ron Earp]

Take a spark plug and beat, pound, drill out the electrode and insulator. Insert a bolt in the plug body, secure bolt with nut, and viola, you have a TDC stop. Use with degree wheel to find TDC. Made one some months ago. That insulation material is hard to get out but with a hammer and punch you'll eventually win.

With the piston to valve clearance I'd suggest some clay and some really light springs. Remove your normal valve springs and replace them with "checker" springs. Then you can push the valve down the required amount to mimic operation.

 

[Frank Catt]

Dont forget to also check spark plug to piston clearance, we have had some that just touch the electrode and close it up, then its a real nightmare trying to figure out why the damn thing wont start. Also figure out flame travel over the piston crown to get the best fuel burn _ it just goes on and on !

 

[dave harris]

To expand on Ron's idea with the spark plug - they are really quite easy to take apart. if you look at the metal base (top part up by the porcelain body) you will see a bulged area that goes from the "nut" area of the metal to the porcelain - just take a grinding wheel or hack saw & cut this away all of the way around. then take a pointed tool (Awl or dental scraper) and scrape the powedered / semi-solid material out. that will leave an "e" ring retainer that you pry out. Now the entire center section of the spark plug will come out of the metal base.

This is an old trick for getting true readings of the "color" of your plugs, as you can check the porcelain at the base without having to use lights / eye loops etc.

Cheers

Dave

 

[Cliff Beer]

The only way I know of to check piston crown to valve clearance is to put the piston at TDC (either measured off the pulley marks, and with a degree wheel for 2-8 or with a plunger micrometer through the spark plug hole) and mark where the valve sits on its stem at full downstroke (you must have all the rockers removed). I typically give it 1mm of clearance at least. My 302 can use 1.7 rockers with a fair bit more clearance than that still but every engine is different.

 

[Terry Oxandale]

It is interesting (depending on the cam profile) where the minimum clearance is between the valve and the piston. I was quite surprised in finding it was well off of TDC (relatively speaking). I used clay with a light spring, zero lash to ensure full valve penetration, and then added about .100 for the exhaust, and .080 for the intake to ensure no interference. I'd not go any smaller than that.

 

[Jac Mac]

Do your check Without the head gasket fitted, that way you will also be checking the radial clearance of the valve head vs valve pockets in piston & it gives you another 0.038"+ safety margin. Checking @ TDC is a waste of time, you must check thru the area about 30° BTDC to about 30° ATDC on the overlap period of cam timing to be absolutely sure & also check all eight cyls, even piston/head manufacturers occasionally get it wrong.

 

[Ron Earp]

you must check thru the area about 30° BTDC to about 30° ATDC on the overlap period of cam timing to be absolutely sure &.

True that!

I learned that the hard way when I was poor (still poor) and in graduate school. Had a Honda twin that someone gave me out of pity and because it wasn't worth much. The cylinder head needed surfacing, badly, as well as a valve job. Got all that done using the barter system with a local mechanic and then checked my assembly at TDC. It was close but I figured it was good to go.

So, I button everything up and a swift kick to turn her over and wham! about blew my knee cap out along with my ankle as the piston encountered the valve. Of course I had to try a couple of more times to make sure the valve was good and bent, after which it'd turn over relatively freely.

What I didn't realize is that I hadn't checked the piston to valve clearance properly and that the head shaving had reduced the clearance a lot as well as slightly altered the cam timing. Took it all apart and yep, valves bent. It stayed apart and was passed onto another poor soul in need of locomotion on the cheap. Maybe he was more skillful.

 

[Richard]

I work better with pictures so here is one I made a while back, like late 60's. It's one for the larger old style spark plug but you get the idea.

If the heads are off use a piece of 1" x 3/8" x 5" stock. Drill a hole large enough for a head bolt on one end and maybe a 3/8" x 1" bolt on the other. Bold the piece to the block with the head of the 3/8" bolt pointing down into the bore some where close to the center of the piston. If you have domed pistons you will not even need the 3/8" bolt, anything hanging over the hole will work.

 

[Wayne M.]

In addition to all the good advice above, use a pair of solid lifters unless you are absolutly sure the light springs you use under the valves are weak enough not to compress the spring in the hyd lifter. Hyd lifters without oil are easy to compress. CAREFULLY rotate the engine through an entire cycle. Remove head and measure the clay. If the clearance is getting close remember the head gasket is not installed so add the head gasket thickness times the rocker ratio to the amount you measured in the clay.
example: .040" X 1.6 = .064"