trying to find true tdc on stock 350(pretty much). i read you can use old sparkplug and 3/8 bolt threaded through it. how long should the bolt be to get true tdc(to stop piston on tdc). no one has one in town, except friend @ machine shop and he said didnt know if it would work because he uses it for dome pistons(homemade). it didnt work, bolt not long enough. what to do, what to do .!.!