Piston fit and clearances

[Greystone]

Guys,

Any input of the following would be appreciated.

I recently fitted a Turbo technics hybrid turbo (T25/T3) to my Rover Tomcat. On Dave Walker's rolling road it did not produce the expected power increase, only 13bhp more than it did with the T25. After some investigation Dave tracked the problem to poor performance on the leak down test (although a previous compression test had suggested that all was well).

I stripped the engine over the weekend and found that I could rock the pistons in the bores at TDC. Pistons are the Accralite T16 pistons 85mm bore 7.5:1 cr (not much skirt on them). By pushing the piston as far to one side as possible there is a gap of approx 0.6mm (24 thou) which indicates a clearance around the top of the piston of 0.3mm (12 thou) between the piston crown and the bore. I removed a piston and checked the bore. The bores have no wear ridge at all. A piston ring inserted down the bore showed a ring gap of 0.4mm (16thou) consistently all of the way down the bore.

I took the head off my other engine (standard Tomcat race series spec - Rover pistons) and found it was almost the same. The piston rocked in the same way (about 0.5mm total gap and 0.25mm clearance all the way around the top of the piston). This engine had the barest of discernable wear ridges at the top of the bore i.e. you could feel a ripple with your finger but it was not enough to catch it with your finger nail. This engine is well used, until I cut them off in order to remove the head it still had the seals from the Tomcat series on it.

It is possible that the leak down problem my be due to some bent valves. The engine came from a Tomcat that was written off at Lydden and the cam belt jumped a few teeth in the crash. I checked the head at the time and a compression test indicated that all was well so I didn't pursue this any further. I did also notice during the mapping that it was fuming slightly from the catch tank when running at max rpm, at although prior to stripping it oil consumption has not been a big issue - it leaks more than it burns.

I have put in a call to Accralite for advice and I waiting for them to come back to me but any thoughts from the assembled brains here would be welcome.

Andrew