LT77 Gearbox, Speedo Drive Pinions needed?

[oldtuckunder]

Hi

 

Trying to sort a friends TR7 V8 speedo readings, they are 15% under.

 

I know I can get the speedo head itself recalibrated, but that means any future failure requires yet another re-calibrated unit.

 

As I understand all TR7/8 speedo heads are the same and interchangeable regardless of what gearbox what ratio rear axle, as all the rotational control is done using different drive gears 7 or 8 tooth in the box and different drive pinions 20 thru 26 tooth.

 

I know that 5% of the under reading comes from the wheel/tyre size being used so I'm trying to work out if I can get it fairly close by just changing the pinion gear.

 

I think but don't know that the internal drive gear is a black 8 tooth. I'm guessing that as the previous owner bought a refurbished SD1 box shortly before selling the car, and I believe (but don't know) that all SD1's used Black 8 tooth gears.

 

Again guessing at the moment that they refitted the TR7 23 or 24 tooth pinion.

 

So I was hoping to locate a 20 tooth pinion which on my calculations would get things fairly close, but seem to be drawing a blank.

 

So wondering if anyone has or knows of someone who might have either a 20 pinion "orange" part no. 219001 or a 21 pinion "green" part no. 219002

 

Alan

 

 

[RogerH]

Hi Alan.

have you contacted any of the gearbox builders. Here is one off the net - http://www.ashcroft-transmissions.co.uk/manual-gearboxes/lt77.html

 

Roger

[cvtrian]

How about trying Steve Wilcox at S&S, or Robsport?

Good luck

Ian

[HowardB]

There are a number of possible causes - the gearbox may originally have not been a TR7/8 box - which means the bevel gear inside the box is wrong. After a lot of playing about with orange amd other coloured gears I gave up and acquired a small corrector gearbox which solved the problem. The box is fitted in the speedo drive cable and it now reads the correct speed.

 

Mine came from Speedograph Richfield, but it was about 6 years ago

 

Cheers

 

Howard

[oldtuckunder]

Yes we know the box is a recent rebuilt SD1 box, so is probably a black 8 tooth, actually removed the existing pinion last week and found that it was only a 22 tooth already, managed to locate a S/H 20T pinion which arrived today, will try it and am hoping that it will be just enough to make the speedo reading sensible although still reading under. Close enough that if the limit is 60 and the speedo says 60 you have nearly used up the margin for error! We will see.

 

Read a neat quick way to alter the speedo if not the distance reading, photo the dial, cut and paste on computer and reprint and reapply to the speedo head, saves messing with trying to find a 19.5T gear

 

Alan

[oldtuckunder]

Wow success! and believe it smack on according to SatNav, well actually consistently 1.5 mph low right through the range so the needle must be about 1deg out of position, but that is very liveable with. Also dropped in the replacement Rev Counter from S&S so it doesn't run 1K below all the time, and whilst in there removed the green dash light bulb covers so you can now actually read all the instruments in the dark. Only small things but completely alter driving inputs. Now all I have to do is sort the headlights on it and then you'll be able to see what your about to hit as well as what speed and rpm you are doing

 

So just in case anyone else in future is crunching the speedo drive numbers.

 

SD1 box (probably black 8 tooth drive gear), 3.08 Ratio Diff, 205/50/15 tyres, a 20T pinion #219001 seems to be what's needed.

 

Alan