piston to bore clearance

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holty
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Tue Nov 06, 2018 11:00 am

engine belongs to a mate of mine, he has had starting problems for the last 6 months getting worse and worse, over the last couple of months, he has replaced stator and carb but these did not cure his problem, he kicked it that much that first gear is goosed so ive got the engine now and ive stripped it , its a gori 175 cast iron kit, he thinks its done about 4 thousand miles since its last piston change, ive removed the barrel and piston, ring gap is big and the bore clearance is big, 0.10mm at the bottom and 0.15mm at the top, that is roughly measured with a feeler gauge, just checking as im thinking its rebore time, thanks in advance,
holty
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Warkton Tornado No.1
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Tue Nov 06, 2018 12:26 pm

Hi Holty.

Presuming the piston is 'conventional' cast & not forged, 3-1/2 thou' (0.09 mm) for an air-cooled cast iron (or linered bore) '200' is the rule that always works for me. A lot of time can be spent getting that spot on, but Lambretta tuning is very time intensive, so honing kit is essential if you have to build fresh Rapido barrels, for instance.

Arguably, although say, Wiseco & others, advocate differing tolerances for forged or very high silicon content pistons, I still apply that rule of thumb & have not had issues ;)

holty
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Tue Nov 06, 2018 7:07 pm

Warkton Tornado No.1 wrote:
Tue Nov 06, 2018 12:26 pm
Hi Holty.

Presuming the piston is 'conventional' cast & not forged, 3-1/2 thou' (0.09 mm) for an air-cooled cast iron (or linered bore) '200' is the rule that always works for me. A lot of time can be spent getting that spot on, but Lambretta tuning is very time intensive, so honing kit is essential if you have to build fresh Rapido barrels, for instance.

Arguably, although say, Wiseco & others, advocate differing tolerances for forged or very high silicon content pistons, I still apply that rule of thumb & have not had issues ;)
yes the piston is an asso ,cast i presume so i am outside tolerance, i think the ring gap looks way too big as well, i said 0.15 at the top but i did manage to squeeze a 0.20 in the top, would this explain his bad starting from cold do you think ?

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garry inglis
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Wed Nov 07, 2018 7:53 am

Definitely holty 0.20 is 8thou

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Scooterdude
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Wed Nov 07, 2018 7:59 am

Its a shame you didn’t do a compression test before you stripped it, I’ve found it it drops to 75psi or below it won’t start.

Yes ring gap far to wide can see just by looking at it .

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HxPaul
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Wed Nov 07, 2018 10:22 am

holty wrote:
Tue Nov 06, 2018 11:00 am
" he thinks its done about 4 thousand miles since its last piston change"
Thats not very far

holty
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Wed Nov 07, 2018 10:27 am

Scooterdude wrote:
Wed Nov 07, 2018 7:59 am
Its a shame you didn’t do a compression test before you stripped it, I’ve found it it drops to 75psi or below it won’t start.

Yes ring gap far to wide can see just by looking at it .
i did do a compression test but it was after my mate rode his bike round here so the engine was warm, compression was very good, which sort of surprised me, but as i could find nothing else wrong thought it could only be a top end problem, i could see the ring gap was massive i was just looking for other opimions to confirm my suspisions

johnny650
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Sun Nov 11, 2018 2:07 pm

the amount of compression lost though the ring gap is absolutely miniscule compared with the overall compression .

if you deduct the area of the over size ring gap from the permissible ring gap the compression loss would be virtually unmeasureable.
An oversize ring gap could never be the cause of hard starting...I seriously doubt if it would even cause slight smoking. Engines start and run fine with broken and missing rings and huge scores in pistons and bores .

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Sun Nov 11, 2018 7:45 pm

piston rings seal the bore not the piston; the piston rings use the lower ring land in each piston ring groove and also the cylinder bore wall to seal the combustion gasses, too big a gap on the rings will cause a massive loss in sealing pressure and the rings can ride back up the ring groove as the piston decends causing a drop in piston ring sealing pressures. The piston holds the piston rings in the correct place but the piston is not what seals the bore.
large ring gaps are not going to help
im not a complete idiot
.
.
s me of the piec s are mis i g

holty
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Sun Nov 11, 2018 8:53 pm

took it to rayspeed yesterday and had it rebored and a new piston, scott and ben where surprised it would start, just need another head gasket to get the squish correct plus a new shim for the gearbox, had a look at the new af kit, looks like a game changer.

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