Technical Replacing Synchronizer Rings

[fiat500]

The gearbox I am rebuilding is an unknown quantity as I have never driven it.
The synchros appear to grab the cones OK but this is static and dry on teh workbench, so perhaps they will slip when under load.
As a subject, it doesn't crop up in online Forums as far as I can see, so it would be re-assuring to know that, as I hope and suspect, it isn't a common fail.
Any experiences out there?

 

[F123C]

Before rebuilding a gearbox, it's a good idea to check all components to see if the gearbox is worth rebuilding from a cost effective point of view!
However, I appreciate that nowadays, with these cars being so old and fewer s/hand 'boxes available, we don't always have the luxury of being able to get a 'good' unit from the nearest Breaker's and have to do the best with what we have.

So.

Synchronizer Rings : It's useful to have a new or relatively unworn synchronizer ring to compare with the used ones you plan to re-use to check for wear.

Points/areas to check :- Outer teeth, these wear to a point and lose the correct shape. The chamfered areas are there for a reason, to help clean engagement with the synchro sleeve, before the sleeve engages with the gear.

Internally, the synchro rings have fine grooves which when new come almost to a fine sharp edge. these wear down over time. the purpose of these grooves is to cut through the oil film and thus synchronize the selected gear speed for quiet engagement. There are also iirc 3 drain slots which can become clogged. So everything needs to be well cleaned.

There used to be a measurement given for the gap between the ring and the gear when engaged. This isn't critical, once there's a slight gap it should work fine. In other words, as the inside of the synchro wears it moves closer and closer to the gear teeth. If there was no gap, the synchro ring would be up against the gear but not able to 'grip' the gear to cause synchronization. You don't apppear to have this problem!

If you're using used parts, you could try different synchro rings on a particular gear to see which is best. You might find e.g. 2nd gear synchro is most worn (or maybe 3rd) but 4th is much less worn. So 4th gear synchro becomes your guide if you don't have a new synchro ring to try.
Some people swap the best condition ring to 2nd gear and the most worn to 4th gear as you can take a little more time changing into 4th.

Don't freak out over all this. As you said in your post, if the synchro ring seems to grip the gear ok dry, it should be ok in service. Just pay attention to cleaning the inside surfaces/grooves in the rings and be careful which oil you use. I don't think EP type is recommended and I'd avoid any gearbox additives, Some of these can clog the grooves I mentioned above, particularly if they have colloidal graphite? in them.

Also, don't forget to check the condition of the teeth inside the (sliding) synchro sleeve which engage with the ring of small teeth on each driven gear. The tooth shape is important , if rounded, it can cause failure to cleanly engage the gear and sometimes jumping out of gear.

Al.

 

[AndrewHarvey]

I had great fun building my fiver speeder - and research backed up the statement regarding the oil selected.
The oil is an essential selection due to the mix of metals within the box. Modern gear oils and old gearbox's are not a great mix
I'll find which i bought and post it up

 

[fiat500]

Thanks Al. That put my mind at rest...very informative. It's strange how I'm stressing so much about this box as my original had very little attention other than cleaning and seals but functions well. I blame this Forum...?

 

[fiat500]

Andrew, I notice that gearbox oil is referred to as "transmission fluid" these days and I guess the EP additives are a rarity.
I read that it needs SAE 80W90.

 

[cambio]

As above, check chamfered outer teeth are not too chipped or worn, the drive flank of the chamfer does the work loading the ring against the gear. Inner ring wiper grooves should be clean and not too worn, and the mating taper face on the gear should be smooth. If it rotates smoothly with a drop of oil and when you press the ring whilst rotating it you can feel it working. Synchro wear gaps are typically 0.15-0.2mm new, check with feeler gauges slipped into the gap.
As far as oil goes, 80 or 90w is your best bet. DO NOT use anything with ep additives, these will eat your Synchro rings and brass bushes etc. Friction modifier additives should be avoided as they tend to reduce friction, the synchro relys on friction to work so you can start to get crashy shifts.

 

[jjacob]

The gear oil NOT to use is GL-5 rated. This is the most available type of gear oil. It will eventually damage the brass and other soft metals in your gearbox, particularly the synchros.
Look for a gear oil of the proper weight (80w90) that is rated GL-3 or GL-4. These will be fine.
I believe that Fiat also recommended straight 40w engine oil which is the same weight as 80w90 gear oil.
I intend to use GL-4 80w90 in my 126 gearbox I am rebuilding.
By the way, the non-synchro gearboxs originally put in these cars can use GL-5.
John

 

[F123C]

When comparing viscosity of gear oils versus engine oils bear in mind that 90 weight gear oil is 'thick' when cold but is about the same thickness/thiness as e.g. 20w50 engine oil when warmed. Try draining gearbox oil after a run, you'll see what I mean.

The 'thicker' gear oil has an effect on easy of gear changing when cold and tends to make the box a litle quieter when warm. It'll never be as quiet as a modern car gearbox, though.

I've used 20w50 engine oil in various Fiat gearboxes.(iirc It used to be allowed). Gearchange is a little lighter but geartrain can rattle at idle speed. I'd be wary of using it in a transmission that included a crown wheel and pinion (final drive in a 500/126) as these gear sets are hard on oil (That's where the EP designation comes from, E.P. = Extreme Pressure).

I'd follow the original manufacturer's recommendations but allow for improvements in oils since then. So a modern oil might be better. If in doubt, you could always contact the Oil Co. Technical Dept.

Straight oils e.g. SAE 40 are often used still in e.g. lawnmowers, generators and other air-cooled engined equipment but they probably don't have additives as found in proper gear oils so I'm not sure if they would adequately lubricate the crown wheel and pinion.

Al.

 

[F123C]

Andrew, I notice that gearbox oil is referred to as "transmission fluid" these days and I guess the EP additives are a rarity.
I read that it needs SAE 80W90.

Many of the modern front wheel drive 'transmissions' do indeed use (auto) transmission fluid (ATF) or something very similar. Viscosity seems to me to be around SAE 10 i.e. very thin compared with 'proper' gear oil.

I always think of gear oil as being runny like honey! :yum:, oops, I meant :yuck:

The reason the term transmission is used nowadays instead of gearbox is because it includes a final drive unit in addition to a gearbox in one unit.

Iirc the gearbox on a 500/126 is more accurately referred to as a transaxle. because it takes the place of an axle, a final drive and a gearbox. So we're now back to gearboxes.

I think the oil used in the 500 gearbox is a compromise. If the final drive (crown wheel and pinion) was in it's own separate compartment then an EP oil could be used here and a non EP oil used in the gearbox part. But because of the reasons stated above by several members EP oil isn't recommended.

The old Austin/Morris/BL Mini had the same conflict. Gearbox in sump sharing the the same oil as the engine. So they compromised by using engine oil for both. Does anyone remember the oil advert that talked about the long chain polymers being chopped up or was it not chopped up by the action of the gears? Sounded good. BL did fit a magnet to the oil drain plug, however.

Al.

 

[IanEmery]

Does anyone remember the oil advert that talked about the long chain polymers being chopped up or was it not chopped up by the action of the gears? Al.

I've got it in my mind that this was Castrol years ago when GTX was first announced? Anyone got a better memory, or is mine correct (which it must be said would be quite unusual)?

 

[cambio]

GL5 contains similar additives to EP oil, to reduce friction and wear but may also harm white metals (brass synchro rings for eg - a lot of more modern transmissions now use sintered materials or even carbon friction faces, so can get away with reduced friction which has benefits elsewhere in this era of chasing fractions of a percent fuel economy and emissions)


Thicker oils, whilst being a bit unhelpful on gearchanges whilst cold, are required as they cling to the gears and protect them under load when hot, particularly in un pumped boxes.
A lot of modern transmisions use atf which generally has a low viscosity but is actually designed to maintain consistent properties across the operating temperature range, as an auto box uses the fluid to control the gearshifts in the hydraulic valve block, shift clutches, torque convertor etc. This is actually at the expense of gear protection properties (thin oil is dispersed from the gear face more easily), however the transmissions designed to use this fluid from the outset have this factored into the gear design, including modules and pressure angles that allow the gears to 'live' with the reduced lubrication. ATF is also full of additives, makes my hands go red too. yuk. I am not a fan!


bear in mind these gearboxes were designed in the 50's and probably earlier, so such technology was just not around then - so any modern oil will perform at least as well as the original fill - probably GL1 - since the base oils from which it is blended have improved thanks to refining technology in the last 40 years.


I have heard of engine oil being mentioned but I believe it harks back to a typo in a Haynes manual! there is no reason to use it in anything other than a mini when a gear oil will offer far greater performance, engine and gearbox oils are very different beasts.
gearbox, transmission, same thing. transaxle = transmission and axle combined, as found on our fiats and other comparable vehicles (Ferrari, Lamborghini etc ), unless you're in the US in which case it can also refer to a front wheel drive transverse gearbox..
transmission fluid - probably sounds better than gearbox oil, hence the price can be upped!
Sorry if I've gone off on a tangent, pet subject
and yes, Castrol gtx or magnetic engine oil, liquid engineering marketing stuff!!