I’m down to looking for every last advantage with my export 85 LG4 305 (non ccc quadrajet).


After replacing the distributor with an Accel unit last year and installing a tiny 421/443 cam I’ve noted the “spread” of timing didn’t please me.

I found an article about making piston stops on motor trend, they made it sound easy and off I went.

https://www.motortrend.com/news/wrenchin-wednesday-build-diy-piston-stop/

Now, what they don’t tell you is how to get the porcelain out of the plugs. If you’re like me, you’ve got plenty of delco plugs at hand. I wish you all the luck in the world “knocking” the porcelain out, I ruined a few trying. It seems delco was pretty proud of these things. The shoulder on the hex must be removed, after which the porcelain can easily be pushed out from the electrode side. I started out with an R44LTS.



They recommend a 3/8-16 thread, I’d go 3/8-24. But I had an exhaust manifold bolt on hand so that’s what I used. Remove the strap, thread the hole (slow, there’s lots of depth) and you’re done.

Now, what I learned about using it.

Remove the ground cable from the battery. Remove all the spark plugs. Use a wrench on the alternator pulley nut to turn the engine - v belt may need to be snugged up if that’s what you’ve got.

Turn the engine over clockwise facing the timing cover such that the timing mark on the balancer is past the pointer on the cover.

Best to go ahead and fully seat the diy piston stop body into the #1 hole. Spin in the repurposed manifold bolt, snug it up. The bolt must be under torque or it will move when the piston contacts it, ruining the measurement.

Roll the engine over by hand until it stops clockwise, observe the position of the timing mark, it should be before tdc. Record the mark.
Reverse direction and repeat, the mark should stop after tdc. Record the marks.

Be sure to read the marks “on plain”, so as not to skew the reading.



After a few attempts, I found cw +4*, ccw -8* wrt the pointer, meaning my “0” mark is actually at 2* retarded. Suggesting my timing light readings would be 2* less than indicated. Indicated 36* = 34* actual if my grade ten math is still any good.


Anyway, I thought this might be helpful.

 

When making my piston stop using the spark plug I simply smashed the porcelain with a BFH. Then used a carbide tipped drill (for concrete) to "mill" the interior of the plug body. The bolt (a 3/8"-16) fit well enough that I keep it in the plug. It's installed as an assembly.
I can only suggest using a light touch when rotating the engine as my concern is gouging the piston top and creating a little burr that might contribute to some pre-ignition. Unlikely but I prefer to err on the side of caution.
Good job finding your correct timing mark.
Some suggest that it's irrelevant and you just have to give what it "wants". Personally, I find it helpful to know the number. It can be invaluable when it comes to troubleshooting.

 

Depending on the cam grind, valve size, head style etc, you should have the rockers for that cylinder backed off to prevent the valves from contacting the stop. A SBC is usually fine since the valves move relatively straight down into the cylinder. A BBC has valves that are canted and will hit a stop when they are opened. Since the valves stay closed with the rockers removed, the stop/bolt needs a hole drilled through it to allow compression to escape.

Many years ago with one of my earlier engine was was trying to adjust timing for performance. Best ET was somewhere around 45-46* total timing. Decided to index the cam only to find out the timing mark was 8* off which put the actual total timing back to 37-38* which is where I expected it to be. Although the mark on the damper is relatively accurate in relation to the crankshaft unless the outer ring has spun, never trust the location of the pointer on the timing cover. I installed an aftermarket adjustable pointer, reset the timing and never had an issue after that.