Hi Bob,
Two years ago I had the rear axle gears changed on my TD mKII (4.875
was the gearing in the MkII) to 4.1 from the MGA. I am most pleased with
the change. Carl Cedarstrand's book said, if I remember correctly, that
the 4.3 was preferred by most owners. I opted for the 4.1 on the basis
that the MkII has the three or six additional raging ponies and could
carry the higher ratio. I note that Moss advocates 4.55.
Just yesterday I had an opportunity to drive a standard TD that had
just been fitted with a Datsun 5-speed. As I am not at all sure that the
instruments are accurate, the 3700 rpm at 65mph may or may not be true.
It seemed to be a workable rig. The owner did not discuss the cost of
having the conversion performed, so I can't compare to my experience.
Perhaps later in the warm weather we can get the two cars together for a
comparison.
Bob
NEMGTR #5547
On Fri, 17 Apr 1998 08:33:43 -0400 "Carolyn & Bob Grunau"
<cgrunau@pathcom.com> writes:
>Hi Art, In my opinion improving the gearing on any pre- 1962 MG is
>worthwhile. These cars were built to perform well on English
>secondary
>roads where acceleration was more important than a high cruising
>speed.
>The introduction of superhighways and our much longer driving
>distances
>has made a higher ratio rear axle gearing very desireable. However,
>keep
>in mind that a stock TC overall gearing is much better than a TD or TF
>due
>to the larger wheel/tire diameter of the TC. Speed in MPH is 15.64
>per
>1000 engine RPM in top gear, so 4,000 RPM gives you 62.56 MPH, not
>bad.
>Changing to a 4.875 ratio gives 16.43/1000 RPM or 65.72 MPH at 4,000
>RPM.
>Changing to a 4.55 ratio gives 17.65/1,000 RPM or 70.6 MPH, possibly
>too
>much for a stock engine.
>You can calculate your own figures using the formula: MPH = RPM x Tire
>OD
>in inches divided by Gear ratio x 336.
>Of course any change in axle gear ratio will affect the speedometer
>reading,
>for a single ratio change down, I don't think its worthwhile to alter
>the
>speedo
>as it was probably optimist anyway.
>Would I change a a stock TC axle ratio just for the sake of better
>gearing
>when driving 2,000-4,000 miles a year? Probably not, unless I had to
>buy
>new gears anyway.
>So here are my comments on the question of how to improve the gear
>ratio:
>The MGA gears and Carl C. book are applicable ONLY to the TD/TF rear
>axle .
> No use for TC except for general discussion unless you are thinking
>of
>changing the whole axle unit ( see below ). I have listed some of
>your
>options below, of course any reference to a TC also includes the TA
>and TB
>as they all use the same axle design. I have driven a TC for 60,000
>miles
>with a 4,875 ratio and found it very satisfactory. When I rebuild the
>car
>again,
>I will use a 4.55 ratio as the more relaxed highway cruising speed for
>our
> roads is justified. I also have a TD with 4.3 ratio MGA gears and a
>TF
>XPEG
>1500 engine, this also works well but again, I would install a 4.1
>ratio
>next time.
>
>To inprove the gearing on a TC you need to either buy a TA gear set (
>4.875
>ratio), a Roger F. gear set ( 4.625 RATIO ), or change the whole rear
>axle
>to something else which has better ratios. Sorry, one more option is
>to put
>a Morris 1000 differential ( 4.55 ratio ) using the TC housing. Also,
>apparently some Ford units fit, but these require special axle shafts.
>
>I chose changing the whole rear axle on my TC to an MGA unit with the
>track reduced to TC dimensions and now have a choice of 5.125, 4.875.
>4.55, 4.3, 4.1 or 3.9 ratios which will all fit the MGA housing. Of
>course
>these last three ratios are totally wrong for a TC. I used the TC
>brake
>back
>plates, brakes, Alfin drums, wheel bearing carrier and wire wheel
>hubs so
>the installation looks entirely stock. The housing, gear unit, axle
>shafts
>and
>wheel bearings are MGA. The MGA axle unit requires a slightly shorter
>driveshaft which I made from an MGB unit. The MGA axle unit gives a
>stronger unit than stock TC, better choice of gear ratios, is less
>expensive for me, and all the components are MG.
>Changing the gear ratio to 4.875 or 4.625 TC gears is a fairly
>straight
>forward bolt-in job, untill you come to setting up the gears. I have
>not
>much experience in setting up a gear set from scratch and don't know
>if the
>Roger F. 4.625 set has any detailed instructions. The TC manual, as
>well
>as other MG books, give some details on how to set the correct mesh
>and
>backlash of the gears.
>Installation of a Morris 1000 4.55 diff unit requires removing the TC
>spider gears and installing them in the Morris diff in order for the
>TC
>axle shafts to fit the splines. My analysis indicated that about 1/8"
>has
>to be machined off the face of the Morris housing in order to get the
>axle
>centerline the same as the TC. However, some installations have been
>made
>without this machining and apparently work OK. You have to drill the
>TC
>bolt hole pattern into the Morris diff and then bolt it in.
>I have no details or experience fitting a Ford diff.
>This reply is far from a complete "how to " instruction and is
>intended
>only to explore some of the options available. Cost and necessity are
>important considerations when making your desision. No doubt other
>options
>are
>possible and I would like to hear of any other alternatives or
>experiences
>from owners.
>Regards, Bob Grunau
>
>
>> To: cgrunau@pathcom.com
>> Subject: TC rear axle gearing
>> Date: April 16, 1998 06:49
>>
>> Dear Bob,
>> Have you had any experience in changing the TC rear axle
>ratio? I'm
>> thinking about getting one of Roger Furneaux's new ring and pinion
>sets
>> while I'm in Europe this summer. I think it's about a 4.8 (but I'm
>> working from memory at the moment, it might be a 4.6). I've also
>got
>> Carl Cederstrand's book on converting the TD/F which has some
>> interesting comments on using a Stewart Warner speedo gear
>modification
>> to get the speedo back on track.
>> Apparently from Carl's comments the main effect is to slow
>down engine
>> wear at "normal" highway speeds (65 mph or so) at the cost of some
>hill
>> climbing ability that might require a bit more frequent changing
>down to
>> third. Acceleration is much less affected, I guess. Since over the
>> next decade or so I anticipate moderate distance driving to GOF's
>etc, I
>> thought it would be a worthwhile modification. I realize that MGA
>ring
>> and pinion sets can be used, but suspect that the setup and
>machining
>> required might be more than what seems to be a "drop in" for Roger's
>> gears.
>> Any thoughts on this? Would you say its worth the bother
>for a TC
>> driven 2-4,000 miles a year (maybe more as I get closer to
>retirement in
>> 10-15 years)?
>>
>> Best regards, Art
>
>
>
>
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