Lost Crankshaft !

[michaeljf]

Hello, You may have seen on various topics here I have been rebuilding a TR6 motor. Well I have just had the FINAL bit of work done on it (Nitrided, checked for straightness and linished) put on the carrier,signed for and it didn't arrive at their depot !!  My question is, is there advantage between a short back and long back crank (It was a short back). Other work done included,crack test,edge of bearing surface checked for "roundness"(no sharp edges to start cracks) 7 bolts+dowel flywheel attachment,re-balance . Regards,Michael.

[michaeljf]

Hello again, Guess what, suppose you may have heard the saying..""Fell off the back of a truck"...apparently it did. A bloke happened to stop and check something in his ute and saw a largish parcel laying on the side of the road (my crank) and saved it ..but had to leave for the USA at 4 AM the following morning so it sat in the back of his ute for three weeks. He found the address of the business where it was checked for straightness (about 20 kms.from where he found it)and returned it there where it is now having a re- check for straightness ! Hopefully I won't have to start again.  Anyone know of any differences between the short/long back cranks (apart from the obvious ).Regards,Michael.

[Nick Jones]

Bad luck followed by better luck!

Hopefully it will be fine.  Differences.... well, I don't consider myself an expert but the obvious difference is that the long back cranks have a longer boss at the rear which moves the flywheel back (I think to allow a thinner, lighter flywheel without changing the clutch measurements).  Presumably TR5 and (some) early PI saloon.  Some of them were cross-drilled as well but perhaps not all.

Short back cranks are more common by far.  Hopefully Mr Thompson may see this and chip in as he knows about these things......

[michaeljf]

Hello Nick, I have another crank (and block but possibly not related) which is short backed and some of the journals are cross drilled. A 2.5 PI sedan motor (long back) which I have used the short backed flywheel (machined about 1.5mm ,larger hub) on.  

[toofast2race]

Hello Nick :-)

 

The difference is 5/8" of an inch or 16mm. All long back cranks that I have seen are crossdrilled and some early Mk2 2500  1968/69 TR6 cranks are crossdrilled also.  Apparently TR250 were NOT crossdrilled but I've not seen one to verify - anyone?  

So all TR5 and all 2.5PI Mk1 are crossdrilled and 5/8" longer.

All Mk2 and TR6 are short - early ones are cross drilled. 

Beware NOS Mk1 crossdrilled cranks - they tend to break (poorly machined factory rejects??)

Edited March 18, 2020 by toofast2race
More info

[Nick Jones]

Thanks for that Andy

Trust all is well with you and yours?

[toofast2race]

All good here despite the current madness! Bit slack for work (temp 40% pay cut) so plenty of jobs going to get done over the next few months  

[michaeljf]

Hello, My engine No.cc25539HE is short backed and not cross-drilled, have another short back that has some journals that are cross drilled and will have to pull a cap off the motor  MG2412HEBW  (long back )that had spent a few klms. in the car to see what it is. Will also give some measurements when I get the wayward crank back. Regards,Michael. 

[michaeljf]

Hello all, Well the wayward crank has returned. A correction to my last post!(it's that long since I had a good look and my declining mind) it is a long back (1 5/8", 42mm) Here are a couple of photo's after all it's treatments and travelling + its flywheel. I had better see if I can start a thread ? for the rebuild. Regards,Michael.

[Nick Jones]

Good news it made it home safe - even if it took a while! 

Long backed alright.  But not X-drilled.  Flywheel has lost a bit of weight!  Is it the original matching one from the that crank?

[michaeljf]

Hello Nick, I don't know if it is the original...but it is the one that was on the crank when I dismantled the motor. (it has a K stamped into it among other things) lots of different things on this motor. Will tell as I describe and attach some photo's in the build thread when I get started. After I finish helping a mate build a shed for a motor-home. You probably noticed the spacer in the flywheel? Will explain later. Regards,Michael.

[michaeljf]

Hello All, Well the thread did get started, and has been a bit ( well very ) long winded. Hence " One step forward two back". The spacer is because I used the flywheel on a Mk2  2.5 PI sedan motor (Auto.)  and is actually rotating again on its original crank! Regards, Michael.