1380 turbo block max bore dimensions
Turbo Mini Forums

I have just fished out a block and stripped it. It was running a MG cam shaft and 73.4mm pistons measured at the skirt.

Now there is very little ware or marking to the piston skirts so I may clean up the heads and look about replacing the piston rings and building my first fresh metal turbo short motor.

My question is what is the maxbore size for a 73.4mm piston If I have to rebore this then its a no go and ill start with a fresh block.

Rob.

Own the day

acrallites are allegedly meant for a 73.5mm bore,

the powemaxes i have are slightly baggy in a 73.48mm bore

Made a call and spoke to John at specalists. Ill be taking the block and pistons to my local engine reconditioner

Own the day

The finished bore size is the piston size at the base of the skirt at 90 degrees across the pin, plus the piston to bore clearance. if you have the bore finished to the bottom limit of piston to bore clearance, the finished bore size is a little under the usual stated bore size. What you may find is that the stated bore size is the piston size plus the maximum recomemnded piston to bore clearance.

So, a piston that measures 73.4mm with a piston to bore clearance of 0.05mm would give a finished bore size of 73.45mm.

with a clearance of 0.05mm (0.002"), you are close to ideal, but, I have, and have others, run a larger than recomended max clearance, with no real problems. The piston will move more, ring seal will be comprimised, and an increase in oil consumption on reasonably high mileage motors is noticable, with out any signs of blue smoke.

Your engineering shop should have the apropriate measuring tools to tell you exactly what the bore clearance is. I was not happy with a clearance of 0.15mm (0.006") while I know certain others were fine with it. The bore should be sized to the pistons being used in it, and it amazes me how many people have the block bored without having pistons first.

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I think I will be taking a standard block instead of the parent block. Hopefully this will save any tears before bed time.

Own the day