Well, not really. The fact you have an OM603 actually gives you a chance to install a tachometer as you have the "missing tooth" on your flywheel. You might even have the pick-up sensor still in its spot.

Sourcing the actual in-cluster tach/clock head might be a problem but it is a lot better than having to pull and replace the flywheel.

Wiring and sourcing the tach amplifier might be your biggest challenge.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now