Gas Engine Magazine

Cylinder Head, Pushrod and Muffler

By Staff

The first repair was the exhaust rocker arm, but before starting, it was necessary to drill out the pivot screw, which had completely seized, even after trying to free it with heat. 

The broken end of the rocker was filed square and then a 1/4-inch slot was cut 1/2-inch deep into the middle of the good metal. A replacement for the broken end was cut and roughly shaped oversized from a piece of steel, making a tongue that precisely fit in the slot. The new tip was then brazed to the old body, and then it was finish-filed for a seamless join. At the same time, a new pivot screw was made together with an adjusting bolt and lock nut for the exhaust valve.

Fortunately, the cylinder head was not warped and did not need truing. The head was clamped on to the milling machine table and the broken valve seats were machined flat. In order to retain as much of the original metal as possible, they were finished to different heights. Each of the old valve stem holes was used as a reference point to set up the head for boring out the hole for the new seat. The holes were then bored out to a diameter of 5/8-inch to provide a shoulder for the replacement seat.

New seats were machined from cast iron. First they were individually taper-turned to marry up with the remainder of the old seat, then shoulder-cut and under-sized holes were drilled for the new valve stems reaming 3/8-inch. 

The method of fitting the valve seats was the next decision. I did not want to use heat on the cylinder head any more than necessary, therefore I decided to press-fit the seats which were made 0.002-inch larger than the bored hole for the last 50 percent of the stem. For good measure some industrial adhesive was used, but this was not really necessary. There was no real pressure on the repaired seats, and the press fit proved adequate. Once the valve seats were in position, the oil holes were drilled and the exterior finish was filed.

New valve stems were made from 3/8-inch silver steel, the ends of which were reduced to 5/16-inch to provide a seat for the old retained valve heads.

Some truing of the valve ports was expected after fitting new seats, and the  valve heads appeared slightly oval. To correct this, the valve heads were skimmed on the lathe until round, as they were thick enough to allow this.

To accurately seat the valves in the cylinder head, up to 0.010-inch of metal had to be removed from part of the ports. To achieve a good fit and remove minimum metal from the valve heads, the sides of each valve head were coated in Engineer’s Blue and the valve slid into position. The blue marked the high spots in the port that were then removed with a small half-round file. This was repeated for each valve until there was a constant ring of blue around the valve port. Then valve-grinding paste was applied and each valve was ground, using progressively finer paste until there was a shiny ring of bright metal on each port and valve.

The valve spring keepers were machined from 1-inch round steel stock reducing to 3/4-inch to slip inside new valve springs, and a 1/8-inch hole cross-drilled for the retaining pin.

A repair to the exhaust threads was necessary, as there was not enough of the original left to grip the exhaust pipe. Cutting 1-inch threads would not be an easy task, as I did not have the right equipment. However, I had some cast adapters for heating pipes, one of which had a 1-inch female thread. 

The exterior of the adapter was turned down to 1-13/32 inches and a hole the same size was bored in the exhaust outlet of the cylinder head, so the new threaded section was a tight fit. After cleaning up the broken section, the thread was put in position, and the top edge of the threaded bush and the broken section of the casting were made good with weld. To finish the repair, the weld was ground down to match the original shape.

Once all this work was completed the head was cleaned, primed and painted silver in line with the original color scheme. At some stage the rocker pivot post had been repaired and I decided to leave this piece of history unchanged.


The igniter was missing, and the original pushrod was not only badly pitted, but bent. As the pushrod was a stock size of 3/8-by-3/4-inch steel, it was an easy matter to replace it. Measurements were taken from the original, and holes were drilled for the cam-follower bolt, trip pivot bolt, catch plate screw and retaining screw for the return spring.

The trip mechanism was also missing, and the age of the engine dictated that a straight trip was standard, but everyone I knew had engines with the later gooseneck trip. Fortunately, Keith’s Associated Engine website ( index.htm) includes two good photos of the straight trip and these were scaled using known dimensions (the size of the pushrod and the distance between the igniter mounting bolts).

A block of 3/4-inch square steel was set in the 4-jaw chuck on the lathe, a 21/64-inch hole drilled, and a 3/8-inch thread cut for a depth of 1-1/2 inches. I turned 2 inches of this bar to a diameter of 3/4-inch and the whole piece was parted off to an overall length of 2-3/4 inches. 

A 1/2-inch cross hole was drilled with a 1/2-inch countersunk oil hole at the top and a stepped mounting bolt made to fit the trip to the pushrod.

A 5-1/2-inch piece of 3/4-inch square steel was turned for 1-1/2 inches of its length, then threaded 3/8-inch. A lock nut was made to complete the trip mechanism.

A new igniter was purchased at a show, but this needed a bit of adjustment to work with the straight trip. The “tongue” on the igniter appeared a little short compared with photos on the Web for straight trip igniters. An extension piece was made to fit the igniter, which was brazed into place and finish filed.

I have to use a battery and coil for the ignition for the time being, as I still have no magneto, so the springs on the igniter had to be set on the left-hand post “always open,” making and breaking when tripped.


A friend’s scrap box revealed a section of steel pipe with a 1-inch thread, which was trimmed to the same size as the original. The old exhaust muffler was badly rusted around the edges but the main body was sound enough. The three retaining rivets were sawed through and the spacing pieces ground off. A pair of 1/16-inch thick steel rings were cut and welded to the thin edges and made good. One-half of the muffler was welded to the new steel tube, and then three 1/2-inch bolts were used to join the two halves with 1/2-inch spacing washers. Primer and a coat of silver paint completed the exhaust.

Tune in next issue for part four, which will cover the flywheel, fuel system, painting and assembly.

Read part one
Read part two

Contact engine enthusiast Peter Rooke at: Hardigate House, Hardigate Road, Cropwell Butler, Notts, England NG12 3AH;

  • Published on Oct 2, 2008
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