Triumph T100 Top End – The Conundrum of Triumph C-Range Top-End Compatibility
Unfortunately, I have chosen to take on a topic that usually results in confusion and misunderstanding. As the resident anorak and customer service guide for The Bonneville Shop, I have set out to compile a timeline for the running changes during the evolution of the unit-construction Triumph T100 top-end.
During the research, I encountered errors in the parts books, errors in a well-known service bulletin, and plenty of dubious information on the internet. While a Triumph T100 top end generally refers to the cylinder barrel, cylinder head, rocker boxes, etc., this article will focus on the relationship between the tappet guide blocks, pushrod tubes, cylinder heads, and the combination of sealing washers that were used over the years to improve top-end oil sealing.
The cylinder barrels changed very little from the earliest unit 500s to the last of the range, in 1974, so I won’t discuss them. The guide blocks were updated over the years, but only three iterations were used during the C-range era, so I’ll keep the details minimal. While there were four different part numbers listed for pushrod tubes, there were really only three types, with very minor differences between the late-model 1969-1974 types.
The cylinder heads are a bit tricky because several part numbers were assigned to the T100C and T100R heads, all sharing common casting numbers, 70-3991 for the early heads, 70-6851 for the later heads, and finally 71-2490 for the final heads with push-in exhaust ports. The sealing washer arrangements changed along with differing specifications of guide blocks, pushrod tubes, and cylinder heads. Where the trouble starts is when the discovery is made that there are components intermingled from different eras of production, often leading to chronic oil leaks in the Triumph T100 top end.
An Overview of Part Numbers and Specifications of the Triumph T100 Top End
Cylinder Heads:
70-4007: 1959-1963 “Squish-Band” chamfer around the edge of the combustion chamber, with very shallow pushrod tube recess machined. 70-3991 casting number.
70- 4785: 1964-1966 Also with chamfered edge around the combustion chamber and bearing the 70-3991 casting number.
70-6966, 70-6700: 1967-1968 The introduction of larger intake valves necessitated a larger combustion sphere, known as the “Hemi head”. Still appeared with the 70-3991 casting number.
70-9507, 70-9509, 71-0277: 1969-1971 Revised pushrod tube receiver depth to accommodate new pushrod tubes, seals, and guide block arrangement. Casting number 70-6851.
71-2643, 71-2644, 71-3605: 1972-1974 Final iteration with unthreaded exhaust ports for push-in exhaust pipes. Casting number 71-2490.
Tappet Guide Blocks:
70-3736: 1959-1963 Inlet & Exhaust
70-4676: 1964-1968 Inlet & Exhaust
70-9352: 1969-1974 Inlet & Exhaust
Pushrod Tubes for the Triumph T100 Top End:
70-3755: 1959-1963 Early unit-construction with large flare at base to accept 70-3758 long lower seal. Thin 70-3547 top seal.
70-4751: 1964-1968 Plain lipped design uses 70-4746 seal retainer cup on guide block with thick 70-4752 seals top and bottom.
70-9512: 1969-1971 Flared bottom and castellated top. Initial sealing arrangement was amended after engine PD 32574 (Nov. 1969).
71-2576: 1972-1974 Dimensionally same as 70-9512, but with oil drain holes in crown. Final sealing arrangement with 71-1283 o-rings top and bottom of pushrod tube.
Pushrod Tube Seals for the Triumph T100 Top End:
70-1497: Paper seal .070” thick, usually superseded to 70-3547.
70-3547: Square section seal .095” thick.
70-4752: Square section seal .125” thick.
70-1496: Square section seal .185” thick.
70-3758: Square section seal .375” thick, long early unit 500.
Service Bulletin Number 324
The service bulletin issued in November 1969 covers a factory “service fix” for the leaking Triumph pushrod tube seals. It is stated that the following modifications are recommended for C-range models after engine number H 65573 (the commencement of the 1969 models):
71-1707: Sleeve, also known as the “wedding band”
70-3547 or 70-4752: Sealing rings, square-section seals, usually clear or white in color measuring .095” or 125” thick.
71-1283: O-ring(s) fitted in bottom flare and on top shoulder of pushrod tube.
It is interesting to note that these modifications were never shown in the 1970-1973 parts catalogs, although the service bulletin states, “Machines produced after engine number PD 32574 have this modification”. PD 32574 is dated as November 1969, so the 1970 parts catalog would have already been published ahead of production of the 1970 models, but why weren’t the subsequent parts books updated?
In fact, the final specifications for the C-range pushrod tube seals and wedding bands only appeared in the TR5T parts catalog for 1974. Meriden did see fit to update this arrangement in the B-range 650 and 750 parts catalogs from 1971-1979. There is also an error in Service Bulletin 324 regarding the white silicone rubber-type seals, where the thick seal is listed as E 3547, and the thin seal as E 4752…the opposite is true.
I hope this informal guide is useful when assembling the Triumph T100 top end on the 500 models, particularly when it is suspected there may be surrogate parts in the mix.