Service Bulletin: Triumph 650 Crankshaft Oil Feed Seal
(Click the service bulletin to enlarge the image)
Howdy C.B.S. readers and enthusiasts. Today I will be decoding another Triumph service bulletin on the two different Triumph unit 650 crankshaft oil feed journal seals. This service bulletin is dated March, 1966. When a Triumph unit 650 crankshaft journal is worn the oil feed journal may become undersized and worn at the same time. When this happens you will need to use an alternative seal. There are two types of seals - standard and +.020". Each seal has a specific application. I will get into more detail on each seal and its specific application below.
Why are there two different oil seals?
(oil feed) Journal on a Triumph 650 unit crankshaft. Notice the slight groove?
When a Triumph 650 unit crankshaft has its journals reground Triumph also recommends having the oil feed journal reground at same time. The oil feed seal over time causes the journal to become worn by producing a groove (see photo). When this happens oil will leak around the seal and shaft which can lower oil pressure to the rod bearings. It is crucial that this seal must be replaced accordingly as oil is fed through the end of the crank. I personally do not recommend having the oil feed journal reground unless the journal shows evident and measurable wear. If your oil feed journal is worn beyond .020", you will have to find a special over-size seal, another crank, or have the end welded and cut back to standard size - not a very common issue.
Standard size oil seal (70-4568)
The first crankshaft oil seal known as the standard seal is kept under part number 70-4568 or E4568. This seal is correct when your oil feed journal is standard. A standard journal should measure @ .622" - .623".
Oversized oil seal +.020" (70-6387)
The second oil feed seal is kept under Triumph part number 70-6387 or E6387. You may also find that some are stocking this seal under part number 70-4568/020. This seal has the same O.D. (outer diameter) as the standard seal but the I.D. (inside diameter) is undersized to coupe with a worn journal. When using a -.020" oil seal your journal should mic out @ approx. .600 - .601". You should not use a +.020" seal if your end journal is on the "standard" size.
For those who might not of noticed - oil seal 70-4568 (E4568) is also the same as the contact breaker seal. We do not recommend using an over size seal when replacing the contact breaker seal. The oversized seal is only to be used on the oil feed journal. This service bulletin was released in 1966 - therefore you can use this same information on Triumph 350, 500 & 750 unit twins.
The next time you have to perform an engine overhaul on your Triumph we recommend that you check your oil feed journal before you start assembly. It's the little details that most builders will over look. In fact, even a Haynes manual or factory workshop manual will fail to mention this valuable information. If you would like to add some input please let us know by commenting below. Thanks for reading!