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Pretty pleased with that...

Technical help for Series one, two and three Lambrettas. Models include the Li, Li Special, TV, SX, GP, Serveta and API/SIL models

Pretty pleased with that...

Postby Adam_Winstone » Thu Jun 09, 2022 2:24 pm

3 weeks before this year's Euro I decided to investigate why my typically ultra-reliable Mugello 240 had started to drip a little gearbox oil, which identified that the alloy inserts that had been welded into the casing, to repair the main M8 exhaust studs, had broken their welds and started to pull out (!). With large plugs having failed, simple helicoils or timeserts where not an option.

With time being limited to resolve the issue, Plan B was to fit a spare iron 200 engine that I had available (not run for a number of years) and try to get the original engine repaired. As it was I did get the original casing repaired, however, by that time I'd fitted the 200, jetted in the carb and put 500 miles of testing on it, with 700 miles by the time I set off for Euro. However, the bit that I'm pleased with...

When I initially refitted the 200 motor, I immediately remembered why I'd taken it out; serious piston slap. I remembered it slapping and having tapped some rings to get a few more miles out of it but it was evident that it was still slapping badly and that I couldn't be setting off for Euro on it as it was.

So, stripping it confirmed that it was a VERY worn 66.6mm piston and bore. Looking around the garage I discovered a 66.8mm piston and wondered if I might be able to sort something out myself, especially as a clubmate had just paid £100 for a 'next size up' rebore with our local automotive machine shop. With the barrel mounted in the vice, a 3-legged honing tool (3 stone sprung type) in my drill, plenty of WD and some feelers gauges, I set about getting messy (lol).

Attentively I set about working the barrel until I could actually fit the 66.8 piston into the bore, then continued until I got suitable piston to bore clearance at all distances along the bore, spinning the barrel around so that I could put the piston in from the head end to allow skirt to bore clearance check. Eventually, when I was happy that the clearance was correct, in a well washed bore, I set about checking the ring gap.

Bugger! Sure enough the 66.8 rings were massively worn and way beyond use. Luckily, a scout through my used pistons came up with a 67mm piston of the same original design 3-ring, and correct ring width.

Nope, they wouldn't fit into the barrel now either so I was going to have to gap these too. Dremmeling off the ring ends allowed me to work 3x rings until they fit the bore, whilst trying to keep the worked ends parallel, then continue working until I had the clearance I was aiming for. Unfortunately, working both sides of the ring ends had completely removed the ring peg location recess so I compressed the rings closed and then worked un a new locator recessed with a round needle file.

Wayhay! Motor fired up immediately after the work, with no evidence of slap. I then put on the 700 miles of jetting (25 PHBL) and running in before setting off for Newhaven, Dieppe and Libramont.

Thankfully, the bike didn't miss a beat and I managed Dieppe to Libramont by 4pm, having landed at dawn that morning. Likewise, I left Bastogne after lunch on Sunday and set out to get somewhere closer to Dieppe for the ferry home, unexpectedly eating up the miles and finding I made the overnight sailing that same evening.

OK, a very 'unstressed' motor spec (standard 200) that could be held flat out, but I was pretty pleased to manage it on a motor that I'd effectively rebored in the garage.


PS - No, I'm not recommending that everyone starts doing their own home rebored... but ;)
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Re: Pretty pleased with that...

Postby Warkton Tornado No.1 » Thu Jun 09, 2022 3:00 pm

Well done, though the use of the three-legged honing tools flies in the convention of what many respected non-lambretta tuners would recommend. However, realising the pitfalls, I confess to having took the risk, but not to the extent that you have.....Yet.

Honing & maintaining the cylindricity is not such an easy task, particularly with the common, short honing sticks that seem to be the only ones available. It's not so much the ports themselves but the spigot that presents the difficulty as the sticks attempt to spring out, so it is a bit of a guessing game IMO. That said, I feel that the best driver is a torquey drill that enables really slow speeds to be maintained smoothly without stalling. There are many drills that will do the job (even in my garage!) but by far my favourite is the Milwaukee M18 combination drill that enables a really good cross hatch pattern to be achieved.

Out of interest, what did you use as a fluid? I used to use cutting fluid as I'd seen that utilised, but paraffin has been the choice of the last few bores that I've honed.

I hope you get around to a top-end stripdown to assess how well you did, because a pattern emerges if any part of the bore has been idle. If all is well, I suggest you rebuild with just two rings as IMHO three are not necessary with a good quality oil.
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Re: Pretty pleased with that...

Postby Adam_Winstone » Thu Jun 09, 2022 3:31 pm

Thanks, yep, pretty pleased.

WD as lubricant.

Yes, I agree about the use of 2 rings only, which is something that I've done with Escort/Mahle 3-ring pistons. I decided against it in this case as I wasn't sure whether this brand (?) Might want the ring aspeg support.

Yep, when I do strip it down, eventually, I'll inspect the bore well for reference indicators.

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Re: Pretty pleased with that...

Postby Solid Air » Thu Jun 09, 2022 3:50 pm

Respect where respect is due there 8-) nice one Adam
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Re: Pretty pleased with that...

Postby Adam_Winstone » Thu Jun 09, 2022 3:59 pm


I do like trying my hand at new tasks.
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Re: Pretty pleased with that...

Postby hullygully » Thu Jun 09, 2022 4:11 pm

now that WAS old skool bud 8-)
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Re: Pretty pleased with that...

Postby Scooterslag » Thu Jun 09, 2022 6:40 pm

Good work Adam, very cool old school 'bloke in the shed' solution. £100 for a next O/S rebore though :shock: jeez that is pricey. Paul
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Re: Pretty pleased with that...

Postby coaster » Thu Jun 09, 2022 8:25 pm

Nice one Adam, I did something similar several years ago when I was experimenting with a reed valved SR 225 barrel. I'd gone through a couple of pistons and was down to a slightly used Racetour piston that would quite fit in the bore so I used a home made honeing tool to open the barrel out to give the required clearance. It worked well but the air leak issues I was having convinced me to cut my losses and buy one of Ron's first batch of Avanti's
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Re: Pretty pleased with that...

Postby Storkfoot » Thu Jun 09, 2022 10:01 pm

Adam, you clearly know what are doing.

I, on the other hand, once tried to hone a barrel with one of those three legged contraptions. It all looked so easy and straightforward but I kept it in the bore too long and ended up with a piston to bore clearance that was all over the place.

My point is that you have to know what you are doing and what the risks are. My experiment was many years ago now. I would put a “don’t try this at home” advisory against using a three legged honer, personally :roll:
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Re: Pretty pleased with that...

Postby dickie » Fri Jun 10, 2022 11:45 am

Nice work. How did you know that the bore was consistent throughout? It must have been hard work making sure you hadn't overdone it in one area versus the others?
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Re: Pretty pleased with that...

Postby Adam_Winstone » Fri Jun 10, 2022 3:16 pm

Yep, that was a big concern of mine so I did little by little, with regular washing out and measuring at different distances in the bore.

I did wonder whether it would be tight slack, tight, etc. As WT suggests, a strip and inspection may show any reference Mark's to indicate.
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