The Yugo engine is based on the Fiat 1100/1300/1500 single overhead cam family introduced for the 128 in the very late '60s. It's an excellent engine, extraordinarily smooth, durable, and capable of extended high RPM use. Part of this is due to the overhead cam valvetrain design and part to the very short stroke that keeps piston speeds down. Because it was intended as Fiat's middle range engine it didn't get the hemi head of the larger twincam 124 engines, but Fiat had years of experience in making simple but effective wedge heads that breathed well.
I've had three of these engines in X1/9s and received good service from all of them. The last one was a little tired but still running decently with over 180,000 miles on it, much of it in heavy traffic, when I sold it to a new owner some time ago. He's a collector and still uses the car on a limited basis. Even wth stock rods and crank a properly built Fiat single cam engine with some additional compression, heavy duty valve springs and a hotter cam will happily turn 8000 rpm. If Yugo had any problem with that engine it was due to their ineptness, not Fiat's original design.
Probably the weakest part of a single cam Fiat powertrain is the gearbox. It isn't bad but as noted in the owner's manual and shop manual oil with hypoid additive must not be used as it will damage the synchronizers. Once the synchos get weak the teeth become damaged and a gearbox rebuild is needed, whereas some stronger gearboxes may go years with bad synchros and just make crunching noises (Alfa gearboxes come to mind, second gear synchro always seems to go early). I would guess that gearbox woes put more Yugos into the scrapyard than engine problems.
I've had three of these engines in X1/9s and received good service from all of them. The last one was a little tired but still running decently with over 180,000 miles on it, much of it in heavy traffic, when I sold it to a new owner some time ago. He's a collector and still uses the car on a limited basis. Even wth stock rods and crank a properly built Fiat single cam engine with some additional compression, heavy duty valve springs and a hotter cam will happily turn 8000 rpm. If Yugo had any problem with that engine it was due to their ineptness, not Fiat's original design.
Probably the weakest part of a single cam Fiat powertrain is the gearbox. It isn't bad but as noted in the owner's manual and shop manual oil with hypoid additive must not be used as it will damage the synchronizers. Once the synchos get weak the teeth become damaged and a gearbox rebuild is needed, whereas some stronger gearboxes may go years with bad synchros and just make crunching noises (Alfa gearboxes come to mind, second gear synchro always seems to go early). I would guess that gearbox woes put more Yugos into the scrapyard than engine problems.
