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Synchronizers let you shift gears without grinding.

1 Overview
2 Warner Vs Porsche Type Synchros
3 Gearbox
4 Part Number
5 Comparison

Overview

Synchro is two cones pressed together until the two shafts run the same speed:

Each gear has a syncho face (external cone) machined with it

The hub is spring loaded so when you move the shifter partway, it presses the baulk ring (internal cone part) against the gear cone.

When they have slowed down to the same speed, the shifter will move all the way in. It happens very quickly, unless the synchros are worn and not working well.

V I D E O Blocker Ring Synchromesh Unit - How it works! (click to view)

Synchro is basically:

Coupling with hub
Baulk ring cone
Gear face cone

Coupling with hub. Left: assembled from the parts shown
Album 22990

Each synchro set services two gears (arrows). Center arrow points to the Coupling Sleeve (The Coupling Sleeve is what the shifter moves)
Album click to view

The hub is keyed to the mainshaft, so the hub sleeve rotates constantly in the Shift Fork while the car is moving.

The spring lets the hub move side-to-side inside the Coupling Sleeve, but not move too far. On this one there is a total of about 1/4" (5mm) total movement possible. The baulk ring, which fits in the hub, is pushed against the gear cone. Once the two shafts are synchronized, the spring pops the dog teeth together.

Cracked shifting inserts can prevent gear shifting
Album 17839

Warner Vs Porsche Type Synchros

F5C56A 5-speed has very different synchros from the 4-speed. It has competition type synchros (the "C" in F5C56A) which are very different: instead of a brass baulk ring, it has two split rings. Both types have a conical section to them. They are the same type as early Datsun Roadster 'servo' type but smaller size.

4-speed: W56 'W' Warner (Borg Warner) BRONZE baulk-ring type syncromesh 
5-speed: C56 'C' Type STEEL Porsche split-ring☁ synchromesh.

4-speed: W56 'W' BRONZE baulk-ring type syncromesh

5-speed: C56 'C' Type STEEL synchro

Comparison
Album 18112

Gearbox

56-Series Transmission

See Transmission Gear Part Numbers

3-speed
32604-18000 B110,B210 RING-baulk
32604-H1000 B10 <> 32604-18000
32604-H7300 B210 > 32604-H1000 <> 32604-H1010

4-speed F4W56L, F4W56A
32604-18000 B10 RING-baulk
32604-H1000 B10 <> 32604-18000
32604-H9000 B120 RING-BAULK (4)
32604-H9000 B310 RING-BAULK F4W56A, F4W60L(F4+F5)
32604-H7300 RING-BAULK B211 F456A, F4W60L (from 7407) > 32604-H1000
32604-H7300 RING-BAULK B210 F456A (from 7407) > 
            32604-H1010 (from 7311) <> 32604-H1000
32609-18000 INSERT-SHIFTING (6) [plate]
32603-18000 SPRING-SHIFTING INSERT (4)
            (or SPRING-synchronizer, RING-SYNCHRONIZER)
32601-18000 HUB-SYNCHRONIZER (2)
32606-18000 SLEEVE-COUPLING (2)
32262 32265-18000 WASHER-THRUST, synchronizer hub 
      [input gear side]

5-SPEED F5C56A
32601-H7200 HUB-SYNCHRONIZER (2)
32606-H7200 SLEEVE-coupling (2)
32611-H7200 RING-synchro
32612-H7200 BLOCK-thrust
32613-H7200 BLOCK-anchor
32614-H7200 BAND-brake 22 (2) $3 USD
32616-H7200 CIRCLIP

Also see 56-Series_Transmission_Parts#Synchros.

32603-18000 SPRING-SYNCHRONIZER 
* B11/B12/B13/B14 4-speed, 5-speed, E16, SR20DE, GA16DE
* B110/B210 4-speed
* B210/B310 4-speed, 5-speed
* N10/N12/N13 F4, F5
* T11

5-speed parts

Album click to view

4-SPEED EARLY
32601-18000 HUB-SYNCHRONIZER 1 4 SPEED $78 USD
32604-H1000 BAULK RING
32606-18000 SLEEVE-COUPLING $52 USD

4-SPEED LATE
32601-H7301 HUB-SYNCHRONIZER, 1ST & 2ND, 3RD & 4TH $83 USD
* same as 60-series Transmission 4-speed & 5-speed (1st & 2nd only)
32604-H9000 BAULK RING $44 USD
* also replaces ealier 32604-H1000
* same as 60-series Transmission 4-speed
32606-18000 SLEEVE-COUPLING
* same as 60-series Transmission 4-speed & 5-speed

5-SPEED
32601-H7200 HUB-SYNCHRONIZER, 2nd & 3rd, 4th & 5th 5-speed
32606-H7200 SLEEVE-coupling

5-speed was never available in pre-56A form (was designed from the go with improvements over the original 56L 4-speed)
Used steel "competition" (Porsche type) synchros in F5C56A form
Datsun Competition in one instance mentioned warner type synchros (F5W56A). However, other Datsun sources such as the B110 and B210 parts catalogs only list F5C56(A) and Porsche type synchros
Datsun Model Series B210 parts catalog (Japan market)
Album 26355

1984 Nissan Competition Catalog:

The 5-speed transmission (FS5C56A) was originally installed on certain non-USA models of the 1200 ... The synchro mechanism is the servo-type (steel).

Album 16819

Pitroad will convert your F5C56A to Warner synchros for 559,000 yen or sell you a kit.

60-Series Transmission

60L vs 60A

#32604-H7300 RING-BAULK
* F4W60 4-SPEED B210
* F4W56A 4-SPEED B210

#32604-H9700 RING-BAULK
* FS5W60A 5-speed (5)

#32604-H9000 RING-BAULK [12-tooth]
* F4W56A 4-speed late (4)
* F4W60L 4-speed late (4)
* FS5W60L 5-speed (5)

63-Series Transmission

A14-equipped B210 5-speed

32604-P0100 RING-BAULK (5) FS5W63A 5-speed
32606-N7100 SLEEVE-COUPLING <> 32606-73200 2
  32605-N9000 HUB-SYNCHRONIZER,4TH & 5TH
  32601-N9000 HUB-SYNCHRONIZER 2ND & 3RD
  32609-73201 INSERT-SHIFTING 6
  32603-73200 SPRING-SPREAD 4
32245-N9000 GEAR-REVERSE,MAIN (38T)
  32258-N9001 HUB-SYNCHRONIZER,1ST & REVERSE
  32608-N9000 INSERT-SHIFTING 3
  32602-32200 SPRING-SYNCHRONIZER 3

32604-P0100 RING-BAULK 
* 63 5-speed
* 71B 4-speed, 5-speed

32609-73201 INSERT-SHIFTING
* 63 5-speed
* Z31 VG30.MT.F5 3rd-4th

32603-73200 SPRING-SPREAD
* 63 5-speed
* B11/B12/B13/B14
* D21 4WD.KA24E.MT.FT
* GC22.Z24I
* N12/N13
* S12.VG30E.MT
* T11
* Z31.VG30.MT.F5

Part Number

3264-18000

32604-18000 B110 56-series 3-speed
32604-18000 B10 3-spd, 4-spd <> 32604-H1000

Per B10 parts catalog, 180 is interchangeable with H10, however the B110 catalog makes no indication of this.

                 H1000  H1001
       Thickness   7.8  8.2 mm
Outside Diameter  69.6  69.3
O. D. less teeth  60.2  59.2

Per Ningbo, the dimensions differ slightly from H10

32604-H1000

56-series 4-speed
* 32604-H1000 Baulk Ring
* 32604-H1010 alternate supplier
* 32604-H7300 > H10

H73 may be used to replace H10, but not vice-versa.

Per Nissan Parts, the original H10 synchros were "replaced" by the #H90 part, however the dimension are different.

Specs for OAT06-551003

aftermarket H10 specs

Some vendors list a 32604-H1001, however it differs from H1000 and it is unknow if this is an actual Nissan listing

                 H1000  H1001
       Thickness   7.8  8.2 mm
Outside Diameter  69.6  69.3
O. D. less teeth  60.2  59.2
cone part min dia   51  50.3
cone part max dia   53  52.3

Inspection

Place the baulk ring over the gear and push down to seat it, then measure the clearance between the teeth. Too little clearance is probably caused by a worn ring.

Normal: 1.05-1.40 mm
Limit:  0.50 mm

Fit ring to gear
Album 22988

Measure 56A synchro clearance
Album 22989

32604-H7300

32604-H7300 RING-baulk (from '74-7)
* 6-tooth: 3 sets of 2 teeth

3 used
* 4-speed F4W56A early
* 4-speed F4W60L A14/A15 gearbox

H73 synchro

Interchange with #H10 is backwards only
Post 479835

Assembled
Album 22970

OPS International OAT06-551002

OAT06-551002 specs

32604-H9000

H90 rings with less dog teeth replaced earlier types that had more teeth. Less teeth makes for easier shifting but is noisier/harsher. The original types made smoother shifts but took more effort on the shift lever.

32604-H9000 RING-BAULK
 replaced #H10 rings
 replaced #H73 rings
* 12 teeth (3 groups of 4 teeth, a group every 120 degrees)
* 60L (not 60A)
  * 5-speed FS5W60L 5 used
    B310 32604-H9000 RING-BAULK FS5W60L
  * 4-speed F4W60L 4 used
    B310 32604-H9000 RING-BAULK F4W60L
* Late 56A 4-speed 4 used
  B120 7510- 32604-H9000 RING-BAULK F4W56A
  B310 32604-H9000 RING-BAULK F4W56A

Album 25677

32604-H9700

32604-H9700 RING-BAULK
* FS5W60A 5-speed late (5)

32604-H9700 Synchro BAULK RING & matching gear surface
Album click to view

Genuine Nissan H97 (60A)
Album 25678

32604-H9700 Synchro BAULK RING

DCIF-2253A (32604-H9700)

H97 has three sets of three internal grooves
Post 479791

32604-P0100

32604-P0100 RING-BAULK 
* 63 5-speed, including B210.A14 versions
* 71B 4-speed, 5-speed
* 30 teeth (3 groups of 10 teeth)

JMIT/Jiun Mu DCIF-2254－32604-P0100/A6410

OPS International OAT06-551008 (32604-P0100)

Comparison

Thickness:
9 mm    60L
8.5 mm  63A/71B
7.8 mm  56
?       60A

Outside Diameter
76.5 mm 63A/71B
69.6 mm 56
68 mm   60L
?       60A

Outside Diameter Less Teeth
66.4 mm 63A/71B
60.2 mm 56
60 mm   60L
?       60A

Inside Diameter
58.6 mm 63A/71B
52.5 mm 60L
52 mm   56
?       60A

Teeth Pattern
3 x 10  63A/71B
3 x 4   60L
3 x 2   Late 56A
30      Early 56A, 56L
30      60A

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Synchros

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