the problem is the ring and pinion and the pinion bearing. Old style original parts are hardend to deep and will break. Also a weak point is the housing .
I did a few zf´s in the past for the M1 procars. They normaly changed ring and pinion after 6 to 8 races . Right now we have some cars they have more than 12 races and the new ring and pinion is ok.
fact is : those cars have 600 to 800 hp and the zf will support this after modification , but the most important thing is to set all the tollerances in the right way.
One point is realy crazy : those zf´s are 40 years old and a good one costs 10 - 15 k !
But nobody thinks about a gearbox service ! When you change the clutch , or replace the engine, please look at your gearbox !!
Often the bearing from the pinion is damaged or the shoulder bearings are not ok. This cause a dramatic gearbox foult.
Here in Austria a gearbox inspection is about 1200 euro new bearings and gasket set you get for an other 800 euro , so total 2000 euro . Maybe 2500-3000 with parts like sychron rings.
(Also new ! We can fix the sychronmechanismen, when the part on the gear is damaged)
Remember : 40 Years ore more old Gearbox !
This maybe only 1200 inspection saves you from a total crash!
I rebuild zfs with broken ring an pinions. When you are lucky a repair like this is 5000 euro or more !
And one point more :
Give your ZF to someone who is professional. Do not open it by your self , do not try to "repair" it.
Professional car mechanics do it better, thrust me.
When I look at me : I start 1995 as a car mechanic, and yes I tried some crazy fixes , but sometimes they didn t work . So my company quaranteed for my work, and I get a chance to learn from my mistakes.
Today I work for the Scuderia Ferrari and do only those cars, wich nobody can do.
My Job is based on my experience with special cars and special parts for more than 25years in this bussines. I worked for ZF , I worked for Ford and since a few years for the Scuderia.
But when I remember my first gearbox, I remember I was in trouble with it.
When I remeber my first differential , I was in trouble !
So please give your ZF to somebody wich have experience with it. Do your job , I do my job. I am no banker, no baker, ec I am specialiced in cars , in motors , in gearboxes since 1995 .
Thanks
Peter