The Stovebolt Site Forums The Shop Area Engine Shop Stovebolt engine builders- - - -heads up!

I made a discovery today that might save some of you engine assemblers a little (or maybe a lot) of grief- - - -or maybe I'm just not as smart as I've always thought! The latter is a distinct possibility, BTW! John and I started reinstalling the reworked cylinder head on the 261 engine in Charlie today, and things were going quite well, at least once a couple of his neighbor guys gave us some much-needed assistance lifting that HEAVY cylinder head back into place. A couple of long 1/2-13 bolts with the heads cut off and a screwdriver slot hacksawed into the top keep the head gasket aligned and also guide the head into the proper place on the block with no damage to the gasket.

Now, here's the "Eureka" moment- - - - -a stovebolt can definitely be an "interference" engine under the right circumstances! The rehabbed head got a slight mill job to be sure the blown head gasket was not the result of a warped head, and all new valves. Once the rocker arms and pushrods were installed, I stated doing a preliminary valve adjustment- - - -get a piston close to top dead center, snug the adjuster screw down gently, back off 1/4 turn, and lock the jam nut down. Move on to the next cylinder in the firing order by turning the fan. WAIT A COTTON-PICKIN' MINUTE- - - - -THE ENGINE LOCKS UP! This happened repeatedly. After a lot of head scratching, and a few choice quotations from the "Old Mechanic's Book Of Words And Phrases" we finally discovered the cause of the problem. Those aforementioned brand new intake valves must be slightly longer than the ones we removed, and unless the rocker arm adjusting screws were backed up several turns, the pistons and intake valves were getting together at the top of the exhaust stroke as the intake began to open. I had no idea that a stovebolt engine could do that! Exhaust valves aren't a problem- - - - -due to the angle they're mounted into the head, there's no way for piston/valve contact to happen, no matter how far the valve is opened.

Fortunately, I didn't use my normal method of adjusting valves- - - - -move the starter linkage to spin the engine to the next cylinder in the firing order. Just one time doing that would have resulted in one or more bent valves or pushrods, or maybe a broken rocker arm. We will be putting the finishing touches on the job in a couple of days, and I'm planning to do a bit of research- - - - -get a piston to top dead center on the exhaust stroke, and use a dial indicator and a pry bar to measure the clearance between the opening valve and the top of the piston. I have a sneaky feeling that it's going to be a lot closer than I thought! Stay tuned for more information!
Jerry

I have read online about the intakes hitting pistons on these engines. What is the proper procedure checking this? Dial indicator on valve at certain point of piston position? Back when I was younger we used to set cylinder heads off of 3406 and 3306 cats by hand. My back says Not Smart.

Similar to Jig timing a Cummins 855 but checking valves instead of injector?

Yes, sort of like the way we would check for piston/valve clearance on the racing engines. The closest piston/valve position comes at the top of the exhaust stroke, as the exhaust is closing and the intake is opening. Stovebolt exhaust valves are not a problem, as they sit in a deep recess in the combustion chamber and it's impossible to touch a piston. Intakes are a different situation, as they sit almost vertical right over the piston. I position a magnetic base dial indicator with the stem on the spring retainer, and as the piston approaches top dead center of the exhaust stroke, I use a pry bar to push the valve down until it touches the piston. Measure the clearance- - - -move the crankshaft a few degrees, and check again, etc. Depending on the cam position and rate of lift, the closest clearance may, or may not happen right at TDC. The rule of thumb for many years has been to have a minimum clearance of 1/16" at the closest piston/valve position, whether it happens at TDC or not. Some cams with high lift rates have the valve outrunning the piston near the top of the stroke. One race engine I built had the closest valve/piston position happening at 15 degrees AFTER top dead center!
Jerry