Britbike forum Forums British Motorcycle specific Brands Triumph bulletin board 6T & 5T crankcase query

Hi all,
I am AussieDerek & have only just joined. I have a major question that I hope someone can answer for me. I am the lucky owner of a 1955 Thunderbird with sidecar that has been in my family since 1959. My Dad also left me a 1952 6T frame & boxes of parts. I was hoping to rebuild the bitsa first before doing a full resto on the family bike. My problem is the 1955 6T engine has been damaged on the timing side & welded some time ago but never reassembled. I have a 1949 5T crankcase with no damage.
The quandary is this, do I try & get the crankcase repaired or can I use the corresponding side from the 5T case. Any help would be appreciated.

Its not perfectly clear form your question, but early cases have a smaller bearing on the timing side (than on the drive side).
Later cases have the same bearing sizes on both sides.
Exactly when the change happened I'll leave to the better informed.

Thank you for that, i will check the bearing position

The 5T used different cylinder base stud spacings (and spigot holes) through to 1955. 1956 on the cases were the same as the 650. The T100, incidentally, used the earlier spacing right to the end of the pre-units. The bearing change came in 1955 for the 5T and 6T, the T100 and T110 got the big bearing in 1954. If you go looking for crankcases, beware that the usual check for big bearing of the "lump" under the timing case is not certain, the casting change came in before the bearing change in some instances, a qiuck ID is the early bearing case had a circlip groove for retaining a chip shield inboard of the bearing. There is one of these on UK Fleabay at the moment.
What is the damage that was repaired on your existing cases? Big bearing 650 type cases tend to go for big money, and do note the difference between the T110 cases with the dynamo drive in front and the 6T that used an alternator. Up to 1953, the 6T (and the 5T) also used the dynamo drive.
Probably as clear as mud, but feel free to ask questions.

I will try & upload a photo but both cranks are stamped with the right engine numbers for either the 500 or 650

Your 49 5T cases will have the dynamo drive on the front of the timing chest (and the small bearing).

I've just had the welding checked & the guy is certain that it's integrity is good. So I will stick with the original cases . Thank you all for your input. I'm sure you will be hearing from me again.

Back again. After checking the weld I had a better look at the cases that I have. The 6T cases do not have a matching cast number like the other sets have. When I compare with my complete 6t , the primary is different. It seems to have the T110 cover. So if I use this one will it cause to much trouble on the other side. I have 2 primary cases, 1 with the removable small cover & the other with the small bump. Also will it look odd if anyone checks the engine number!!!

I guess you mean that the numbers stamped on the lower front are different, that's what identifies unmatched cases. As long as the barrel and sump plate surfaces are flat, generally not a problem.
Primary chaincases are a whole nother thing. The cases for rigid frames are longer, with two bosses at the upper rear of the inner, Early rigid types had the teardrop casting in the outer, as used on models with a dynamo. For the 5T in '53 and the 6T in '54, an alternator was fitted inside, and this had a small cover up front of the outer. Note that the T110 was always swinging arm frame, so had the shorter covers, but again a dynamo, so the teardrop shape.
You would do well to find a copy of "Triumph Twin Restoration" by Roy Bacon, which details a lot of this stuff (I don't try to remember)
Whatever, all of these chaincases fit all of the pre-unit crankcases, the three screws that hold them on remained the same.

Thanks TinkererToo,
I have ordered that book. Both of my bikes are rigid frame. My problem is I have the 52 frame, 1x650 engine & 2x500 engines all in boxes. They have been in storage for 15-20 years & in no order. My 6T with the outfit that has been in the family since 1959 & I am trying to match that bike with the bits that are have, the 2 engine numbers are only 600 units apart.

The '55 Thunderbird should be swinging arm, the '55 model first started deliveries circa August '54. If you have the engine/ frame numbers (even partial), I should know the score! Might be some delay, as I'm off to my workshop, but there are plenty of others who could help.

I have checked all of the numbers with the Triumph club. My Thunderbird outfit is complete & has been in the family since 1959 is a rigid frame built in 1955 & even has an “R” after the frame number. It has a matching engine number.
The other frame is a 1952 model (rigid). The engines I have are a 650 1953 model, a 500 1949 model & a 1953 model 500.
When I match my 650 engine with my complete bike the timing side is different but they are only 600 units apart so that makes me think that they are a mismatched set. So, do I try & get the correct timing side crankcase or just make do with what I have. How will that affect the rest of the build.

That '55 was obviously a factory special, thus the added R, I guess they could do that if they still had the older parts on hand. What the rest of the spec for the engine etc is anyones guess. The earlier type engine would have a circlip groove in the timing side crankcase bearing housing for retaining the chip shield used with the small bearing. As I said earlier. the test for matching halves is the stamped number on the lower front of each case half. Putting pictures on this forum is a bit of a pain, you have to save them to a web address so they can be linked, but it would certainly help here!