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Dick

Gudgeon Pin Offset

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I've got a project on the go at the moment that involves an engine rebuild using County pistons. They are Triumph 2000 + 0.030".

Now, measuring the pistons, there is a 0.020 offset on the gudgeon pin. There are no front arrow orientation marks on the piston crown. So which way round does the shortest side of the offset go? I think it is to the thrust side of the cylinder. The engine is conventional clockwise rotation.....looking from the starting handle end, so the thrust side would be on the left. So short side of the piston to the left??

Any advice gratefully received.

Regards,

Dick.

 

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I may be completely wrong here Dick but to be honest I can't see that a half mil offset is going to make any difference at all so it doesn't matter which way round they go, how on earth did you pick up on such a marginal thing? 

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I was always taught any offset in the gudgeon pin from the piston centreline should be set towards the major thrust side of the piston.

Digging out one of my reference books "Complete Automotive Engine Rebuilding and Parts Machining by Robert Scharff,  I see it confirms  " A pin offset (it can be up to as much as 0.062 thou) is designed to reduce piston slap and noise that could result as the large end of the connecting rod crosses over upper dead centre,"  so that's what I would do also.

Mick Richards

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Surprised there's no arrow to the front on your pistons. I'm currently rebuilding a 2500 saloon engine for my GT6 with County +20 pistons and there is a forwards arrow on each crown.

Suggest you call the supplier to check.

Nigel

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59 minutes ago, Nigel Triumph said:

Surprised there's no arrow to the front on your pistons. I'm currently rebuilding a 2500 saloon engine for my GT6 with County +20 pistons and there is a forwards arrow on each crown.

Suggest you call the supplier to check.

Nigel

Nigel,

Could be used pistons that have been “decked” for compression height.

Mick Richards

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Thanks for the replies.

The offset not only helps to reduce piston slap but even though its a small amount helps the piston 'roll' over TDC and BDC enabling higher revs with less stress on the conrod. If you draw out the geometry it is more understandable. There is a fair amout on the net that I found this afternoon. 

Another way of getting the same effect is to bore the little end offset and not offset the gudgeon pin. 

I attach a photo of the project.....6 cylinder, 3000cc, 105bhp as standard at 4500rpm. Most spares are 'make it yourself'!!

The engine rebuild included....rebore to Triumph 2000 pistons, Kent cam with better profiles....Brabham Viva, Arrow conrods with Spitfire bigend shells....two side by side in each, remetal mains, balance everything, mild porting and raise compression ratio to 7.5:1.

The expectation is around 115-120bhp and more torque.

I dont know why County do not mark these pistons with a forward facing arrow, perhaps the off center oversize stamp is supposed to indicate to fit on the thrust side and maybe only split skirt pistons have arrows. I've tried to find County's web site but no luck yet.

Onward ever onward.

Regards,

Dick.

 

20190519_201108.jpg

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2 minutes ago, Motorsport Mickey said:

Nigel,

Could be used pistons that have been “decked” for compression height.

Mick Richards

They have been 'decked' by me, no markings prior to machining

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