Cylinder Bore Dia.

[JohnRoberts]

Hi Guys  Does anybody know the actual bore dia. to suit +.020 pistons

Thanks  JohnR

[Motorsport Mickey]

Normally I'd measure the pistons across 2 planes (they are oval, not round) and then allow about 3 1/2 thou running clearance above the largest dimension. Compare against the specified piston/liner clearances in the workshop manual, that normally defines bore size.  In use that clearance can easily wear by a couple of thou and still give acceptable performance...even spectacular, race engines are machined to maximise performance by reducing drag just before the wear starts to erode the fits between piston rings and bores. 

To compare against any liners you may have already, measure the liner bore about 2" down from the top with internal snap gauges (telescopic expanders) and then measure that spread with a micrometre, make 2 or 3 readings to allow for differences in your feel as you get used to doing it. I've used an internal micrometre in a similar fashion but the restriction of the bore means your hands are often in the way, so not so easy .After a bit of practice with the gauges you'll get proficient at selecting the actual axis of bore by eye and develop the feel as you twist the expanded gauge through the 90 deg plane to obtain the actual bore dimension. Within half a thou is achievable, note your readings and average them.

Having undersized liners machined to suit oversize pistons may not be as cheap as you think if that is your plan, and may make it uneconomic. 

Mick Richards

[JohnRoberts]

Hi Mick.  Thanks for your reply. I have measured all 6 bores they round with next to no taper, dia. measures 2.964'' { + .020 above std. }

               The pistons [ marked +.020 ] that are fitted measure 2.944'' dia. at the top   So the clearance is 0.020'' ring gap is 0.027''

                Regards JohnR

              

[Motorsport Mickey]

Sounds good, as long as there is no wear lip and you are happy with the honed surface they look to be goers.

Mick Richards

[Dave McDonald]

Brown book Workshop Manual gives bore as 2.94" so plus 20 thou should be 2.96".

Dave McD