By David Porter, Customer and Technical Support at The Bonneville Shop
From time to time, we receive reports from customers working on piston ring replacements on their Triumph engines and the measured ring end-gaps appear to be out of specification. There can be several reasons for this perception.
Sometimes, the technician performing the job simply doesn’t know the size of the pistons presently in service in the engine. Usually, oversize pistons used in a cylinder barrel that has been bored will have a designation of the oversize stamped on top of the piston, for example, .020 or .030.
Once the proper measurements are made, and the cylinder bore is deemed to be serviceable in its present condition, sourcing piston ring replacements is pretty straightforward for the given engine model and oversize. There are occasions where there are no identifying numbers found on top of the pistons. Typically, if there is a part number, such as 70-9488, found on top of the piston, it denotes standard bore condition.
The fun begins when there is no identification whatsoever on the piston. Measurements will need to be taken to determine which piston ring replacements are sourced for the job – this is essential for piston ring replacement. Without knowing the bore size and piston diameter, sourcing the piston rings is a guessing game, and will likely result in an improper fit of the piston rings, manifested by the illusion that the end-gaps are too wide. Triumph issued a Service Bulletin, number 11/71, addressing the preparation and fitment procedure for replacing piston rings in the “B” range engines on September 20, 1971.
The bulletin is very succinctly written, and although it doesn’t go into great detail on using the instruments necessary for obtaining the measurements, it does provide clear instructions on evaluating the cylinder, pistons, and establishing the piston ring end-gaps. Since this service bulletin was intended for dealership service technicians, the detailed measuring procedures during piston ring replacement were assumed to be known and practiced in the service department by certified Triumph mechanics.
“Now, wait a minute, you may be thinking…the factory workshop manual states that the fitted gaps should be .010-.014”, so what figure do I use?”
In the bulletin, it is stated that the correct end-gap is .012-.018”, measured with a feeler gauge at the lower end of the bore, about an inch from the very bottom of the cylinder. (Using a piston to square up the ring in the bore is useful.) This gap should be observed regardless of the oversize of the pistons and bore. In other words, the gaps for the rings should be .012-.018” on a standard bore cylinder, as well as a cylinder that has been bored to .060” oversize, using the appropriate .060” oversized pistons.
Now, wait a minute, you may be thinking…the factory workshop manual states that the fitted gaps should be .010-.014”, so what figure do I use? Given the characteristics of modern fuels available to us today, I am suggesting that the service bulletin recommendation of the .012-.018” ring gap is more valid than ever.
There has been much written about this topic over the years and other more technical reasons for the various opinions on piston ring end-gap. Interestingly, an earlier Triumph Service Bulletin number 323, issued October, 1969, states that “machines from engine number DU44934 on 650cc models and engine number H49837 on 500cc models, the piston ring gap recommendation has been changed from the original figure of 0.010 to 0.014 to the revised figure of 0.015-0.020”.
One general guideline that has been in practice for many years with piston ring replacement, is that ring-gaps can be established by gapping .004” for every 1” of bore size. So, using a 76mm Triumph 750 piston, approximately 3”, we would establish a .012” ring gap. One very well-known and respected Triumph engine building shop on the west coast suggests a gap of .0045-.006” per 1 inch of bore for the Hastings top compression ring and oil control ring, with a gap of .0055-.007” per 1 inch of bore for the lower compression ring.
Do the math, and you come up with a range of .0135-.018” for the top compression ring and oil control ring, and .0165-.021” for the lower compression ring (middle ring on the piston).
“One general guideline that has been in practice for many years, is that ring-gaps can be established by gapping .004” for every 1” of bore size.”
So to summarize, the task of fitting new piston rings can get as technical as you allow it to, but it is really a simple procedure, and the Service Bulletin 11/71 is a good guide for this undertaking. Do not be alarmed if you are checking ring gaps on a new set of piston rings and see a gap of .016”, where you are looking for .012”.
The iconic Bernie Nicholson is quoted as recommending piston ring replacement when the end-gap exceeds .030”, measured at the unworn bottom part of the cylinder, so you will have a long way to go before your rings are worn to the point of replacement if you start with a gap of around .016”. Another respected Triumph enthusiast, John Healy, is quoted as saying “the hotter the engine runs, the more ring gap is required.”
More on Bernie “JB” Nicholson
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