Military recon numbers + Series petrol bore sizes

purplemk

Member

Posts

75

• Mar 18, 2016

• #1

Hello
I have a Series petrol engine, ex military recon and it has a plate attached giving information about the reconditioning, can anyone help me decipher the code letters and numbers please?
Also what is the standard bore size for a 2.25 petrol? I have done a few searched but found some conflicting figures so if anyone has a proper manual with the correct STD size that would be helpful
Thanks

 

jamesmartin

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91,879

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louth lincs

• Mar 19, 2016

• #2

hi std nominal size is 3.562 so plus 20 is 3.582 but because tolerances vary from nominal they can be several thou above or below,yours has plus 20 pistons and a standard crank which is 2.500 mains 2.312

 

Last edited: Mar 19, 2016

purplemk

Member

Posts

75

• Mar 19, 2016

• #3

jamesmartin said:

hi std nominal size is 3.562 so plus 20 is 3.582 but because tolerances vary from nominal they can be several thou above or below,yours has plus 20 pistons and a standard crank which is 2.50 mains 2.32

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Thanks for the quick reply
That makes sense then because my measurement of the bore's gave a size of 3.583 / 3.584, However I am concerned to know if they actually fitted +20 pistons as well because if I push the cylinder side to side (still installed in engine) there seems to be quite a bit of free play between cylinder and bore. I have not yet actually measured the pistons as I was hoping not to have to disturb the bottom end however just eyeballing it the piston slop seems considerable.

 

jamesmartin

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91,879

Location

louth lincs

• Mar 19, 2016

• #4

pistons are measured at 90 degrees to gudgeon pin bottom of skirt ,pistons arent round till they get up to temp,you need to measure 90 degrees to cam just below top ring travel ,when you measure your doing it cold ,piston may have more movement,but measurements will tell, if skirt is 3.854 but piston 3.582 its ok

 

purplemk

Member

Posts

75

• Mar 19, 2016

• #5

So by the book what would the ideal clearance be between piston and cylinder wall?
I assume on the plate that the S is for "standard" mains and big ends as you explained, pistons should be +020"
What about the numbers in the next column that run vertically? any ideas what they stand for?
Thanks

 

jamesmartin

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91,879

Location

louth lincs

• Mar 19, 2016

• #6

o.oo17 to o.oo22 thou
serial number used by the reconditioners

 

purplemk

Member

Posts

75

• Mar 19, 2016

• #7

jamesmartin said:

o.oo17 to o.oo22 thou
serial number used by the reconditioners

Click to expand...

Ok thank you

 

purplemk

Member

Posts

75

• Mar 23, 2016

• #8

I have now had a piston out and measured it and it seems to be in good shape. The piston measured at the skirt and at 90deg to the cam I get 3.580 and the cylinder bore is 3.583, the bore is consistent up and down etc
Question, I assume it is normal for the piston to be slightly conical? It measures a few thou less at the top than at the skirt, I always assumed they would the same size top and bottom so what is the reason for the conical shape?