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1300/1500 piston clearances


sparky_spit

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Would someone who has access to a proper workshop manual, rather than just the Haynes I have here, be kind enough to look at some specs for me please?

For the 1500 engine, piston to bore clearance is given as 2 to 3 thou top, 0.12 to 1.6 thou bottom.

For the 1300 (with very slight differences for different piston manufacturers) the clearances are 20 to 25 thou top, and 1.9 to 2.4 thou bottom.

I'm wondering why the huge difference between the top clearances for 1300s and 1500s? Especially as the 1147's clearances are quite small, similar to the 1500's?  Has haynes missed out a nought, or put the decimal place in the wrong place?  I want to get this right, before I hand over block, new pistons and clearance specs to the engineering company.  I know they will want to use their own judgement too, but depending upon which set of specs they pick up, there's a world of difference between 25 thou and 3 thou. Thanks in advance.

PS - pistons will either be County, or possibly AE Hepolite. I notice that new County pistons tell you where to measure the piston dia, but not what they recommend for clearance!

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Hello Sparky,

many misprints, 0.12" is an impossible clearance for a piston as is the 20 to 25 thou. Even the ring gaps will be a lot less. Your machinist should have the necessary literature?

As an aside, pistons tend to neither be round or parallel, particularly the top lands, which decrease in diameter from bottom to top to compensate for the relative running temperatures. Full diameter will be across the thrust sides and slightly smaller at 90 degrees to that.

Alec

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piman wrote:
Hello Sparky,

many misprints, 0.12" is an impossible clearance for a piston as is the 20 to 25 thou. Even the ring gaps will be a lot less. Your machinist should have the necessary literature?

As an aside, pistons tend to neither be round or parallel, particularly the top lands, which decrease in diameter from bottom to top to compensate for the relative running temperatures. Full diameter will be across the thrust sides and slightly smaller at 90 degrees to that.

Alec


Yes, the machinist was talking about 2 thou clearance, but wants to see the pistons first. He'll also bore each cylinder to match the pistons should there be any differences between them. He just wanted to confirm clearances with me/workshop manual.

And yes, using a digital vernier on both old and new pistons, they are definitely oval (widest diameter at 90deg to the pin) and they taper upwards.



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