TV Cylinder Fit

[Littlewheels]

Took my TV175 cylinder to an engine rebuilders for a rebore. The company is used by several Lambretta enthusiasts and also a dealer so when we discussed tolerances I asked them what was the norm and they said 4Thou when I was thinking more 2.5thou as suggested in the book.

I bought the cylinder for my engine rebuild at the last Melton Parts Fair for a very reasonable £50 with piston and head but after a closer examination I found a hairline crack in the spigot! Got this welded by a specialist and they did a grand job so a lot of work time & money invested in this already.

This week I was ready to start thinking about fitting the cylinder to the engine so I chamfered the transfer ports and then proceeded to fit the rings in the bore and check the gap which was 20thou!

Cylinder has been bored out to 63mm which equates to 197.92 circumference
minus 63.1mm = 198.234
= 0.31449 converted to imperial = 0.01238
So based on the difference and what the gap is at 63.1 then at 2.5thou the gap would have been 20 thou minus 0.00619 = 0.0138thou which would also have been too much?

Makes me wonder if the rings are the right size and more worryingly if the bore to cylinder tolerance is too much??

[Storkfoot]

Are you using an old used piston?

[lofty]

in my mind the best and most accurate way to check is with micrometers on the piston and bore..

the ring gap method is great to judge wear on an existing set up, but for me there are too many randoms in using the rings to judge bore size..

ie, ring circumference and its tolerance
whether it sits completely circular in the bore or is tilted slightly
accuracy of the feeler gauges
and if you are turning circumference into diameter there is a 3.14 (you may be using more decimal places for pi) multiplying factor that exaggerates any tolerance in measurement ..

this is mostly my conjecture, so happy to be corrected (everyday is a school day )

[Littlewheels]

No it's a new piston

Not trying to establish bore size here as it has been measured properly and is 63.1mm

What I want to know is if the clearance 0.1mm = 4 thou is too much?

What do you think is the best clearance on an original cylinder and piston?

My workings are to find out what the gap would be with 0.05 clearance.

Thanks

[Storkfoot]

I am fairly certain mine was bored to 4 though although I asked for 3 thou. It may wear out quicker but it has been perfectly fine and revved freely, without a hint of seizure, during initial running in.

Personally, if you know the person who bored the barrel to the piston has a good reputation and if the piston is from a known manufacturer, I’d just put it all together and try it out.

Out of interest, what make piston do you have and what squish do you have? I ask that last question as mine was 2.3mm on the original cylinder head and barrel

[Littlewheels]

Thanks Storkfoot

The piston i'm using is a Borgo original 3 ring.

The squish I haven't considered yet but I think i'll go for something like 1.75 - 2mm

Yes think i'm going to run it as it is and see what happens

[Storkfoot]

Thanks Storkfoot

The piston i'm using is a Borgo original 3 ring.

The squish I haven't considered yet but I think i'll go for something like 1.75 - 2mm

Yes think i'm going to run it as it is and see what happens

Please let us know what squish you get. I would have had to get a new cylinder head machined up to reduce mine any further.

[dickie]

I had a 125 rebored by af a couple of years ago. Ben advised 4thou to make running in easier on the assumption that I wouldn't be doing many miles and it was only a local runabout, so therefore I didn't care too much about longevity.

[Adam_Winstone]

Sorry, my mind doesn't relate to thou so cannot immediately comprehend your clearance. Also, my tried and tested piston to bore clearance measurement follows the old feeler gauge in situation method, rather than measuring piston and bore independently. Both of these mean that I can't immediately comment on your figures, however, I can tell you a little relevant history / story...

Well ahead of Milan 97, a clubmate sent his original 175 barrel off to a respected main dealer for a rebore, allowing plenty of time for running in. Said dealer stated 4 thou was appropriate and duly rebored to suitable piston and returned in good time.

Bike reassembled and running in intended. However, the rebuilt engine appeared to be unusually noisy so it was stripped and checked. Nothing untoward found so reassembled and.... still noisy, sounding like it could be crank, bearing or top end related (piston slap, crown touching head, other) so stripped again, with pal having new crank checked and top end going back to dealer that rebored it. Crank OK. Top end returned as 'nothing wrong with that'.

Reassembled.. noise still there!

Top end stripped and driven to another well respected dealer, who measured it an immediately said that the bore was worn out.

"Can't be, that's a new piston and rebore!"

"I can see that from the machining and honing marks but it is bored too slack and what you're hearing is piston slap."

The dealer went through his cupboard, found the next oversize and rebored again. Once assembled the motor was wonderfully quiet, run in appropriately, then did Milan and back, via South of France, without issue.

When my pal went back to challenge the first dealer he stated that it could have been run like it was, without issue. His justification was that they bore them all like that as scooterists cannot be trusted to run a barrel in!

[Littlewheels]

Thanks for all the replies.

So now I may consider a rebore especially as I've got another piston. I have waited so long to use this engine but I also want it to run as it should.

Adam can you tell me what clearance you have on your cylinder/piston or should I ask what size feeler gauge fits between the two when its been machined correctly?

Also who did the second rebore?

[Adam_Winstone]

MSC did the 2nd (correct) rebore.

I typically work to the tolerances that Mark has been good enough to post on MB website / documentation. You may want to double-check what he's published but something along the lines of:

Wear limit to rings (in bore, towards top, where rings run) = 0.6mm

Piston to bore wear (piston in bore, suitably towards top so that you can measure between bottom of skirt and bore wall) = 0.15mm

If you can fit that feeler gauge in the gaps above, you're at or beyond the wear limit.

NB: Others will argue the above, which is fine for them but working to the above has served me well.

PS - I have regularly sent others to MB for oversize and conversion rebores and, to date, had nothing but excellent work done.

[Littlewheels]

Thanks Adam,

I think you are correct but I also think that maybe yours friends rebore although being told it was 4 thou clearance was probably a little more than this especially if he was getting piston slap.

Today I dug out an old tv 175 barrel that I vapour blasted a few years ago with standard 62mm bore. Looks in great condition with no score marks, maybe slight evidence only of where the rings ran at the top of the cylinder.

Placed an original not Borgo piston in it and measured the gap with feeler guages = 0.0035" My newly rebored cylinder has a gap of 0.0038" so I am going to use the new barrel as it is I think with the worst case scenario being I will need to get it rebored to the next oversize.

I'll let you know how It runs.............

Thanks all for your help