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TR4/4A 3A and 4A Clutch Component Differences?

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K

karls59tr

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I notice that Moss has different part numbers for 3A and 4A clutch discs. Also different #'s for pressure plates between the two models. Same with release bearings. I was under the impression that the clutch setups were the same for the 3A and 4A. Does anyone know what the differences are or is it just different stock numbers for ordering?
 
i can't answer why they have different part numbers, but my experience is with the pressure plate that has the three lever arms. That's what came with my TR3A when I bought it brand new in 1958. I remember changing the fingers about 1964 and again in 1992. Also at least once, O used my MIG welder to build back the rounded end and grond it back to where it was usable for another 20,000 miles or so. Then I bought a new full kit from Roadster Factory with the same three fingers. I find it has always engaged very smoothly.

I once drove a TR4A and as I was letting the clutch out, nothing happened - then all of a sudden, it engaged (or should I say - it clamped suddenly) and I prefer mine with the three fingers.

I have heard that the owners who race their TRs prefer the sudden type of clucthing action for a quick start. I have also heard that some replacement clutches for TR4 to TR6 with the multi-star shaped relese levers are from Saab. And these are heavy to cope with the 150 HP and more that these clutches need to have.
 
Basically, I believe the difference is between the (older, as on 3A) coil spring clutch

coil-spring_clutch.jpg

and the later (as on the 4A) diaphragm clutch

diaphragm_clutch.jpg

NOTE: These are generic images from out there on the Interweb thingie, but hopefully they illustrate the two types.
 
Thanks for the info. The clutch set up in my 4A has the diaphragm clutch the same as the 3A! Perhaps it was changed out by the PO? or maybe a case of "they used what they had on the shelves at the factory" type situation that you hear about.
 
Karl,

You mean that someone has installed a diaphragm clutch in your TR3A?

Scott
 
Maybe some more photos will help. Here is a TR2-4 clutch (shown with a centering mandrel in place):

clutch_zps8a42362a.jpg


And here is a TR4A-TR6 clutch. There were several minor variations, but they all had lots of little fingers (the "diaphragm" since they are cut from a single sheet of metal) instead of the 3 big fingers on the earlier clutch. The TOB, pressure plate, clutch plate and flywheel are all different.

2UsedLaycock.jpg
 
HerronScott said:
Karl,

You mean that someone has installed a diaphragm clutch in your TR3A?

Scott
Click to expand...
Easy enough to do, but you'd have to put in a TR4A flywheel or re-drill the TR3 flywheel to match the bolt hole pattern of the diaphragm pressure plate. I had to do that (use a TR4A flywheel) when I put in the Toyota gearbox in my TR3A, as Herman's kit requires the use of a diaphragm clutch cover. I presume because of the hydraulic throw out bearing that he uses.
 
Basically all the TRs had an old fashioned spring type clutch until the 4A when Triumph changed to a 1/2" smaller diameter spring diaphragm clutch. It's better and lighter to operate, so a common modification to the earlier cars along with a lightened and balanced flywheel. I've done mine.

if you decide to modify your car you'll need to change the thrust race and the carrier that it fits to. This are in the Moss catalogue.
 
EV2239 said:
if you decide to modify your car you'll need to change the thrust race and the carrier that it fits to. This are in the Moss catalogue.
Click to expand...
Frequently you will also need to trim the front cover of the gearbox slightly (about 3/8" IIRC). Otherwise it will fit fine at first, but when the pedal is depressed the clutch fingers will hit the front cover.
 
karls59tr said:
oops! I meant to say I have a coil spring clutch in my 4A similar to my 3A.
Click to expand...
Should work fine, as long as you remember to order the TR4 components when you need to freshen the clutch. To switch back, you'll need to make all the changes outlined above; except I'm not certain about redrilling the 4A flywheel for the larger TR4 clutch. BTW, that is a machine shop operation as the locating pins need to be pretty accurately located. If the clutch is not precisely centered, it will make the assembly off balance.
 
Karl,

So someone has gone the opposite direction. :smile:

So they could have put a TR3/TR4 flywheel on the original TR4A engine or perhaps swapped a whole TR4 engine including flywheel for the original engine. Does the engine have a TR4A engine number?

Scott
 
Engine numbers were typically within a few hundred of the car's commission number and TR4A commission numbers went from CTC50001 to CTC78684 (live axle TR4A's were marked CT versus CTC) so I would expect a TR4A engine number to be in that same range. Mine have the original engines according to the BMIHT certificate and here are those commission and engine numbers for comparison.

  • VIN: CTC69123L
  • Body: 68668CT
  • Engine: CT69238E

  • VIN: CTC72159LO
  • Body: 71922CT
  • Engine: CT72497E

TR4 commission numbers went from CT1 to CT40304 so the TR4 engine numbers should be in that range.

Scott
 
Last edited:
So no prefix on the engine number? That's different than the earlier cars.
 
Randall,

I'll double check but I'm pretty sure that I did not see any prefix on my TR4A's when I originally documented them before the getting the certificate. My TR3A does have the prefix on the engine number as you mention.

  • Commission: TS51339L
  • Body: EB49811 (1054832)
  • Engine TS51386E

Scott
 
No big deal, Scott, I was just surprised. The chart at https://club.triumph.org.uk/prefix1.htm seems to indicate that TR4 and 4A motors used the CT prefix, but I never paid any attention myself. And it certainly wouldn't be the first time that the cars don't match the documentation.
 
4A CTC58457L has CT prefix on body, engine and gearbox which are all within 400 of commission #.
 
Randall and Andy,

So much for being pretty sure. :(

Andy hit it on the head for the once-restored original TR4A where the CT were hidden by the coil bracket. The second unrestored TR4A had the CT hidden by the rust-preventative system (ie thick layer of oil and grease) that I hadn't cleaned off well enough when I was documenting the numbers. :smile:

I updated my Word document and the post above!

Scott
 
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