Rebore questions - *updated with pics*

[Piatti]

Some advice on rebores required please

A 150cc barrel comes as 57mm standard, rebore sizes go up 00.2 mm
So oversizes are:

57.2 - 1st oversize
57.4 - 2nd
57.6 - 3rd
57.8 - 4th
58 - 5th
58.2 - 6th
58.4 - 7th
58.6 - 8th
58.8 - 9th
59 - 10th oversize

How far can you rebore a 150cc barrel ? could you rebore to 175cc

A new 175cc barrel comes as 62mm standard

Would a 10th oversize 150 barrel (59 mm ) be approximately 160cc

If you did rebore to 10th oversize the piston would be 20mm bigger than standard size, could you still use the original 150cc cylinder head

Would you have to change carb settings – main jet size

Regards

Piatti

[tea5ive]

you can go from 150 to 175, this is a popular and common conversion but you will need your head machining to suit.

[bristolmod]

and yes you will need to make sure your jetting is OK.

Whenever I go from 150 to 175, I tend to put a 20mm Delly on jetted for a TV175 to start and work from there.

Having said that, I'm running an LI150Special with a 64mm piston giving me c186cc, and this has been kept on the original 18mm carb, with the original LI150Special jetting- I've checked the plug very carefully and it runs superbly well and has done for the last 3000 miles (touches wood and exits stage left....)

Chris

[Piatti]

Thanks for the replies,

Its a 150 special am getting rebored, did a compression test yesterday and it was below 100 psi.
The barrel has been rebored before, but I can't remember to what oversize.

I want to keep running the original 18mm carb.

If I rebored to 10th oversize 59mm the piston would be 20mm bigger than standard size, could I still use the original 150cc cylinder head

If not whats involved in machining the head to suit

Regards

Piatti

[Spanish Fly]

20mm

2mm not 20mm.

Check its not touching or too close to piston. If it is, dress it back by hand. A decent half round file or scraper will do it if used with care. be carefull not to damage the gasket face.

SF

[Piatti]

20mm

2mm not 20mm.

SF

Ha, my mistake

[Piatti]

Took the head off and piston out, the piston is 57.8 - 4th oversize, only discovered this after cleaning it up
This was the state of the piston



This was the original 150 head


I think the problem was the thin metal head gasket was not trimmed to match the rebore of 57.8, this left a slight lip
on the gasket. You can see were the piston has been hitting the right hand side of the gasket on this pic below.


Checked the ring gap as compression test was low, ring gap was over 1mm
Maximum gaps are 0.6mm for over 125cc engines

[goldeneye]

clean it up, new rings, jobs a good-un. piston looks alreet to me. you would gain fook all taking it to 160cc.

[warts]

Agreed. Didn't you start off wanting a reliable daily driver?
And anyway, that compression isn't bad for an old 2T. A light hone/glaze buster session + new rings. Likely better than many new ones.

[tonydevon]

? thats 110 psi isnt it? not below 100