By Dave Porter, Customer and Technical Support at The Bonneville Shop
Table of Contents for triumph top end sealing
Let’s talk about the Triumph top end. When we think of a typical air-cooled British vertical twin-cylinder motorcycle engine, I would propose that the majority of us think of Edward Turner’s iconic Triumph twin. The external rocker boxes sitting atop of the cylinder head really are an impressive feature, and those beautifully chromed push rod tubes are a pleasant visual contrast to the black painted cylinder barrels and the aluminum head.
While BSA and Norton aficionados might argue their twin-cylinder engines are just as iconic as the Triumph, they do have bragging rights over the Triumph twins when it comes to keeping the top ends oil-tight.
History of Triumph Top End
Neither the BSA nor the Norton twins employed external push rod tubes, but rather featured cast-in tunnels in the cylinder barrels for the pushrods, which are the nexus between the cam followers and the rockers. This is not to say that BSAs and Nortons don’t ever leak oil from the top end of the engine, because they can, but it’s not due to pushrod tubes. Designers and engineers at Meriden spent years revising the tappet guide blocks, push rod tubes, and oil seal arrangements in an effort to keep the leaks to a minimum.
Despite all of the revisions and improvement in oil sealing, the problem persisted all the way until the end of production in the early 1980s. Given the fact that many of these motorcycles have been on the road for upwards of 60 years or more, the resulting services required to keep these old Triumphs running have undoubtedly contributed to the complexity of sealing up the push rod tubes, due to milled cylinder heads and incorrect or incompatible parts fitted out of desperation.
Resources
Fortunately, today we have the resources in a strong aftermarket for service parts, improved machining equipment, and plenty of access to information via the internet. Many Triumph parts suppliers have technical pages on their websites, as well as experienced staff members willing to offer guidance to customers. There are a number of excellent online forums where topics like push rod tube leaks are frequently discussed.
Original factory parts catalogs are an indispensable reference tool when deciding which combination of tappet guide blocks, push rod tubes, and oil seals are to be used for your given model year Triumph B or C-range engine. It would be too easy to just select the part numbers from the parts book for your 1969 Bonneville without any regard to what may have been retrofitted to the top end and what milling has been done to the mating surface of the cylinder head, in an effort to correct warpage. Instead, a careful examination of the entire triumph top end of the engine is required to determine which parts to order, and how to proceed.
Considerations For your Triumph Top End
“Simply put, you do not want too much or too little push rod tube crush on your Triumph top end.”
Some aspects to consider are the proper “crush” on the push rod tube seals and o-rings, the amount of head gasket surface remaining after a head has been machined to correct warpage, the thickness of the tappet guide block shoulder (where the lower seal sits), the overall length of the push rod tube, as well as the depth of the machining in the cylinder head, where the top of the pushrod tube locates. Of course, the combination of upper and lower push rod tube seals is critical but can’t really be determined until the overall collection of top end parts fitted is surveyed.
Due to the interchangeability of various model year parts, a Triumph 650 engine could be fitted with a later set of guide blocks, but with early unit-construction push rod tubes and cylinder head, or vice versa. Or you could be working with a cylinder head that has had the head gasket surface completely decked to nil. This doesn’t render the head unusable, but it has to be dealt with if an oil tight top end is desired. The other risk to consider using a head in this condition is excessive push rod tube crush, which will bend the head when the bolts are torqued to specification.
Sometimes a thicker than stock head gasket is called for to achieve the optimal push rod tube crush of .030”-.040”. The stock copper head gaskets for the B-range Triumph twins are .050”, but thicker versions are available in .080” and .120”. The head gasket thickness not only affects oil sealing but can be used to control warping of the head, the effective compression ratio, and piston crown/valve interface. Simply put, you do not want too much or too little push rod tube crush on your Triumph top end.
Push Rod Seals
Once the top end assembly is underway, and the compatibility of the components has been deemed acceptable, you can consider which thicknesses of push rod tube seals are best to start with to calculate the resulting crush. I keep an old .025” thick late T100 head gasket (70-4675) for measuring crush, using it like a big feeler gauge. Here’s a list of push rod tube seal part numbers and their thicknesses:
- 70-1497 .070”
- 70-3547 .095”
- 70-4752 .125”
- 70-1496 .185”
- 70-7310, 71-1283 O-rings .103” cross section (1969-later)
Various combinations of these seals can be used to achieve the desired gap of around .030”. Many practitioners of vintage Triumph motorcycle service keep an inventory of gaskets and push rod tube seals on hand, as they cost little, but can save waiting for the ideally sized seals to arrive from the supplier. While this blog is not intended to be a definitive step by step instructional article, my goal is to share some information that is not covered in the workshop manuals. Thanks for reading and happy wrenching!