From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Testing Specialloid Pistons in a Bentley engine, detailing performance, issues with knocking, and a subsequent piston seizure.
| Identifier | ExFiles\Box 148\4\ scan0316 | |
| Date | 11th February 1935 | |
| L1001 11.2.35. Specialloid Pistons. We have now completed a test with Specialloid Pistons in Bentley and find that they do not give any improvement over Aerolite pistons as far as power is concerned. The maximum power developed with a standard engine having 1.400 dia. inlet valve and 1.300 exhaust valve and a compression ratio of 6.42:1 was approximately 111.0 BHP. with a maximum B.M.E.P. of 116.7 at 2000-2500 rpm. The complete weight of pistons with rings and gudgeon pin was 20 1/4 ozs. and the weight without rings and gudgeon pin was 15 ozs. This is a reduction of from 2 to 2 1/4 ozs. on the latest Aerolite pistons as intended for production. The engine at high speeds appeared to be smoother than with Aerolite pistons due to the reduction in weight. Several of the pistons knocked when cold and we should say at both top and bottom of the stroke since the knock was reduced somewhat by cutting out the cylinder but did not entirely disappear. The skirt of these pistons is tapered rather drastically, there being only .002 clearance at the bottom of the skirt and .0055 at the top, which shows a taper of .0035. With such a large clearance at the top of the skirt and with the pistons protruding considerably from the bore at the bottom of the stroke, it will be realised that there is every chance for the pistons to knock when cold. At the end of the power curve whilst running at 4500 r.p.m., No. 1 piston seized so that we were forced to discontinue our tests. The seizure was probably due to the small clearance at the bottom of the skirt. We have found on Aerolite pistons that we cannot reduce this clearance to less than .0025 without running into the risk of piston seizure. We did intend subjecting these pistons to our usual endurance test for some data with a view to reducing still further the weight of aerolite pistons, but the seizure of No. 1 piston prevented this. HS{Lord Ernest Hives - Chair}/R.Matthews. | ||