FIAT 780 handbrake disc replacement

[mungus]

Hi - my first post! :newhere:

Has anyone here ever replaced the hand brake disc on a late 1970's FIAT 780 (Heston)?

Originally they had a pair of thin discs fitted, (that usually buckled and even broke; as I discovered mine had). FIAT have long since superseded them with a single thick solid disc (pic attached). Its even made in 2 halves, so that theoretically one doesn't have to split the tractor in two to replace it. Just remove the side cover / lever housing etc, and slip the 2 halves over the park brake gear and bolt em up! Or so my FIAT parts dealer told me...

What a great idea I thought! Well done Mr FIAT! So I looked forward to replacing the busted OEM discs with the correct 26 tooth thicker single disc.
(there are 3 types of disc, so I counted the teeth before I ordered and checked the disc matches; it does)

Problem is even though the disc halves simply won't go over the hand brake gear no matter what I try. I even have far better access and views than would be the case usually, as I already had the body off the tractor, and the splitter case's top plate has been removed. You can't slide them over from the rear side as I imagined, as there is a thick washer (retaining ring) with a bigger OD, blocking it. And the gap between the front face of the gear and the splitter casing is too narrow too... (photo and cross section from manual attached). I even got desperate late one night, and tried drifting one half over the edge of the gear, which worked for one half but not the other and also resulted in damaging some teeth on my lovely new disc!
I still can't get the other half over and that really can't be the way to fit it!

As my tractor is still sans body I am about to split the beast in two and remove the bevel gear etc, per the OEM disc procedure. Yuk!
Can anyone tell me what I am doing wrong? I am good mechanically (do all my own work) and have a decent workshop, but this ones got me stumped!

Is there some magic way to get that big obstructing washer off and back on again in situ? Maybe a FIAT/Heston mechanic can share their knowledge?

 

[mungus]

An update in case in helps anyone else.

I have now fitted the new thicker hand brake disc. (photo below) It did require splitting the tractor and removing the bevel drive nut. This allowed enough free-play in the handbrake disc gear cog to slip the 2 halves over and bolt them up. This is not as my FIAT parts agent claimed; that you can fit the 2 halves over by only removing the side plate / lever arm housing. However even this is a big improvement, as the original one piece (thin) discs required removing and refitting the bevel driveshaft (full days work after removing the platform, so call it 2 days for most of us).

Where the claim by FIAT parts may be correct is for those with the bigger handbrake discs. (There are 3 sizes; 26,27 and 35 tooth cogs, each corresponding to later models of 780/880). The reason being is that the bigger discs are bigger than the bevel shaft, so they should fit over the bevel shaft and slide up onto the disc cog.
Many of the smaller / earlier 26 teeth discs will have the problem I encountered, (although the very first ones may be OK). The reason being that after frame number 847335 a packing ring was fitted behind the disc cog, the OD of which is bigger than the ID of the disc halves. This is what prevented my disc halves slipping over the disc cog.

So there you have it, for my 780 you need to remove the platform and split the frame to fit the new disc. The good thing being that the new disc is so thick and strong it should outlast the tractor...