Rebore questions - *updated with pics*

Anything related to Lambrettas... ask tech questions, post helpful info, or just read and learn.
Piatti
registered user
Posts: 65
Joined: Wed Jan 07, 2009 6:11 pm
Location: Deeside, Flintshire
Contact:

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
Last edited by Piatti on Thu Jun 27, 2013 8:24 pm, edited 1 time in total.
User avatar
tea5ive
registered user
Posts: 575
Joined: Sat Mar 21, 2009 3:41 pm
Main scooter: cometa 140
Location: gilberdyke, east yorks
Contact:

you can go from 150 to 175, this is a popular and common conversion but you will need your head machining to suit.
bristolmod
registered user
Posts: 1741
Joined: Wed Jan 07, 2009 8:42 am
Main scooter: Lambretta TV175 S3- what else!
Location: Bali-Hai bar, Locarno Ballroom Bristol, 1967!- mines a Brown Split!!
Contact:

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
Scootering since 1968.
Piatti
registered user
Posts: 65
Joined: Wed Jan 07, 2009 6:11 pm
Location: Deeside, Flintshire
Contact:

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
registered user
Posts: 198
Joined: Tue Nov 30, 2010 1:23 pm
Main scooter: P200e
Contact:

Piatti wrote:20mm
2mm not 20mm. :oops:

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
registered user
Posts: 65
Joined: Wed Jan 07, 2009 6:11 pm
Location: Deeside, Flintshire
Contact:

Spanish Fly wrote:
Piatti wrote:20mm
2mm not 20mm. :oops:

SF
Ha, my mistake :oops:
Piatti
registered user
Posts: 65
Joined: Wed Jan 07, 2009 6:11 pm
Location: Deeside, Flintshire
Contact:

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
Image
Image

This was the original 150 head
Image

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.
Image

Checked the ring gap as compression test was low, ring gap was over 1mm :oops:
Maximum gaps are 0.6mm for over 125cc engines
Image
goldeneye
registered user
Posts: 1037
Joined: Wed Apr 01, 2009 2:28 pm
Contact:

clean it up, new rings, jobs a good-un. piston looks alreet to me. you would gain fook all taking it to 160cc.
warts
registered user
Posts: 7092
Joined: Thu Aug 04, 2011 6:28 am
Main scooter: honda
Location: Fenland
Contact:

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
registered user
Posts: 553
Joined: Wed Jun 08, 2011 12:26 pm
Main scooter: '71 GP125
Location: Northam, Devonshire
Contact:

? thats 110 psi isnt it? not below 100
heavy is good, heavy is reliable, and if it does fail, hit them with it!!!
Post Reply Previous topicNext topic
  • Information
  • Who is online

    Users browsing this forum: No registered users and 19 guests