[The following article is reprinted without permission from the Rocky Mountain Vintage Racing Ltd newsletter, date unknown. Thanks to Kirk Buecher (kirkb@gr.hp.com) for submitting it.]
TECH TALK
by Milo Muterspaugh
The four cylinder Sunbeam engines are a nice design with their 8 port aluminum head and high intake ports. Unfortunately, these engines are prone to failure of number 2 or 3 rod bearing even when used on the street. This article will outline some modifications that have resulted in reliable race engines when operated up to 8000 RPM.
The key to a reliable engine is the installation of 350 CI Chevrolet connecting rods. The Chevy rods are .075 inch longer than the Sunbeam rod and may be used with slightly modified stock Sunbeam pistons. The steps required to use the Chevy rods are:
1. Obtain 4 matched 350 rods with 3/8 inch bolts.
2. V8 rods have one side of their big end machined off so that they are narrower than the sunbeam's one inch. With the cap bolted to the rod, have the cut off side heli-arced with bronze to a height greater than one inch. The bronze doesn't need to be very uniform and it is best not to braze across the parting line between the rod and cap.
3. Using a stationary belt sander remove the minimum amount of metal parallel to the rod bolts so that the Chevy rods will fit in the Sunbeam cylinders. Don't remove any more material than is required to get the rods through the Sunbeam cylinder, so if the engine is to be rebored take this into account.
4. Have the bronze surfaced by an automotive machine shop to give .010 inch side clearance on the Sunbeam crank.
5. The wrist pin hole needs to be honed approximately .030 inch to float the Sunbeam wrist pins in the Chevy rods. The clearance should be .0005 inch and an oiling hole should be drilled to oil the wrist pin like on the Sunbeam rod.
6. Install new rod bolts and recondition the big ends of the Chevy rods.
7. Balance and glass bead the rods.
8. Grind the rod journals on the Sunbeam crank to 2.100 inch to provide .002 inch bearing clearance. Use Clevite 77 type Chevy rod bearings.
9. If using stock pistons, cut .060 inch from the top of the pistons. This will give a deck height of .010 inch if the block hasn't been surfaced. This modification will result in a full point increase in compression ratio with the 1725 pistons. Using the 1600 pistons with .060 inch removed will result in an even higher CR. If custom flat top pistons are to be used, have the compression height set to 1.790 inch for an unsurfaced block.
1. Groove the center main journal on the crank the width of the groove in the bearing shell and approximately .060 inch deep.
2. Drill the oil passage from the main oil galley to the center main all the way with a 21/64 inch drill bit.
3. File the oil pump housing to provide no more than .001 inch of end play for the pump gears.
4. Obtain a new oil pressure relief valve or shim the old one to provide 60 psi of oil pressure. These valves are also prone to sticking if the pistons are too tight.
5. Be sure and remove all the oil galley plugs and thoroughly clean the block. Don't have the block cleaned at the machine shop without also cleaning it thoroughly yourself before assembly.
Of course, the previous should be augmented with good high performance engine preparation techniques. Also, to make power above 6000 RPM, you'll need some cylinder head work and a high performance camshaft.
Milo Muterspaugh
Colorado Springs