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Rebore before big end?
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JustinW
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January 1, 2020 - 9:50 am
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I'd like to get a rebore and new piston as it is a fairly straightforward process not requiring removal of engine. I have a larger piston ring gap than spec. I've seen some comapnies offerring this service by mail order recommended on here.

However, I do have some play in the big end, and will probably get this sorted next winter.

Any technical reason why I can't get the rebore done, run it for a season, then get the big end done? Or will the big end play mess up the new piston? 

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SpacedMarine
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January 1, 2020 - 12:14 pm
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JustinW said
I'd like to get a rebore and new piston as it is a fairly straightforward process not requiring removal of engine. I have a larger piston ring gap than spec. I've seen some comapnies offerring this service by mail order recommended on here.

However, I do have some play in the big end, and will probably get this sorted next winter.

Any technical reason why I can't get the rebore done, run it for a season, then get the big end done? Or will the big end play mess up the new piston?   

Are  you sure you cant just get away with new rings?They do wear.

As regards the big end which way is the play?Up and down or side to side.Reason being up/down = bad a little side to side is acceptable. 

What's 7/16 in mm again?

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mikef
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January 1, 2020 - 4:24 pm
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Hi Justin.
    Just my thoughts on this.
Piston ring gap out of spec probably won't cause you too much trouble, if starting and general performance is not too bad.
However big end with excessive up and down play could get nasty and let you down in quick time. Just depends if the engine was noisy or had bad vibration.
Sometimes best to get things sorted in "one hit" then you don't have to worry.
I can't think of a technical reason that the big end would adversely affect the re-bore, just that it will save you stripping it down again next season to replace the big end.
Mike.

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AdrianS
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January 1, 2020 - 4:24 pm
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A touch of vertical play can be left until it gets worse if it is only a touch.

you need to measure the piston to barrel clearance to determine if just new rings will help. You may get some new rings only to find the gap is still too big.

a reborn and new piston should give greater power/compression which will speed up the wear of worn big end.

sounds like a case of an engine rebuild over the winter months for the spring!

not too difficult to remove the engine and strip it.  Will also benefit by fitting new oil seals and bearings if necessary. 

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Sponge
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January 1, 2020 - 4:48 pm
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If big end play gets too excessive the piston stands a chance of crashing into the cylinder head. This would certainly mess up your new piston. 

Suggest get somebody from the club nearby to you who knows how to check your big end bearing and cylinder bore for wear and meet up to take some measurements.   The head and barrel will need to come off to do this.  Once you know what is out of tolerance and what is OK you can make a decision about how to take things forward.      

Any play in the big end will get worse quite quickly. 

Sponge 

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JustinW
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January 1, 2020 - 5:24 pm
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Thanks for thoughts everyone.

I did put new rings on, but being a newbie to this didn't hone the barrel (I didnt know about this and I haven't the tools to do so). 

The new rings also had out of spec ring gap, plus there is a little step at the top of barrel which is unswept. So, a rebore would be justified.

But I just went ahead with engine as is. It does run, and doesn't make a nasty rattle or banging. It is however underpowered I feel.

So I'm left with rings that won't bed in due to lack of honing. I'm thinking take off barrel, get it bored out +20 with new piston and see how it goes for not a lot of money. And then, end of season do the big end and seals. 

If the bike starts making a racket then new big end immediately ..but this seems like a much bigger job. The play is up  and down, and I suppose it isn't too terrible as I've driven about 90 miles now without noticing anything which sounds nasty. But power does seem low, so maybe try the rebore?

It seems like throwing money away to get barrel simply honed without rebore, and I probably shouldn't carry running rings which will never bed ..and this lack of bedding might be reason for lack of power. Bit of a quandry.

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Stoo63
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January 1, 2020 - 5:29 pm
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Sounds like a very sensible way to proceed to me, Justin. Good luck with it. Do you need advice about an engineer to rebore it for you. Must be one down your way.... Someone will know.

Happy new year

Stewart

'52 D1 direct lighting plunger, '58 Square Four (project), '59 D1 direct lighting plunger,  '59 Tiger Cub, '60 5TA,  '76 FS1-E

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JustinW
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January 1, 2020 - 5:54 pm
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Hi Stewart, thanks, I had this company bookmarked ** Please log in to view **

to post barrel to. Haven't spoken to them yet.

Some people recommended them on the forum I believe. But I'm happy to receive other recommendations localish to Exeter/Newton Abbot etc or by post.

Happy new year.

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nickjaxe
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January 3, 2020 - 8:33 pm
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The big end will deteriorate rapidly...and be very unpleasant to ride...with lots of vibrations.

Get recommendations on how is ok for 2 stroke re-bores....not everybody is...not as simple as a 4 stroke.

What part of the country are you, Justin.

You could do with measuring your bore accurately...is it noticeably worn??

Nick.

My Bantam video              https://www.you.....jpOFmzRZRI

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JustinW
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January 3, 2020 - 10:59 pm
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Hi Nick. Well, yes the bore was noticably worn in that a) there was a ridge on the unswept part and b) brand new rings had an excessive end gap double the recommended max.

I've now sent barrel off to PJ engineering who will bore to +20 and supply Rex Gaunt piston. They seem to know 2 strokes well and have done many Bantams. I'll report back on how it went. It's around a 10 working day turnaround. As to big end play, I'll keep a very close ear to it. The play is there but on checking again I'd say the play is tiny, and hence why I've felt no vibration as yet.

I'm down in hilly Devon. I'll certainly notice if there's an increase in compression with rebore. Fingers crossed. 

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nickjaxe
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January 3, 2020 - 11:20 pm
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My big end failed and went from smooth the very buzzing in the seat and handles bars during a 15 mile ride...its not so much an ear job...more a feel

Have you only just started using the bike after a long lay off.

Is PJ the place in Wolverhampton...if it is they should be good.

But you big end won't improve...I was unlucky...when mine went B175...rex was waiting a good few months for a new batch to come in...a guy near me in Chester fitted it...did the re-bore as well...he only works on 2 stroke....re-builds them for the racing community....

But last we heard is out of action with a neck prob....I have no doubt if you rang him he could recommend somebody to re-build your crank...he is very very good.

Nick.

My Bantam video              https://www.you.....jpOFmzRZRI

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JustinW
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January 3, 2020 - 11:34 pm
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Yes PJ in Wolverhampton. Yup, engine has been laid up for a long while. I replaced one oil seal which turned out to be fine when removed it, and visually inspected another which looked ok too. It's all a learning curve though!

If the bike runs better after rebore I'll be looking to completely rebuild crank and replace all oil seals. I haven't got any yardstick to tell if engine is under par or just normal. I've managed 32 mph max speed slightly downhill on gps, I'd have thought it would do a bit more. As to inclines , speed drops drastically, almost embarrassingly slow.

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JustinW
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January 3, 2020 - 11:36 pm
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...but don't get me wrong, I'm thoroughly enjoying the whole thing!

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mike p5xbx
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January 3, 2020 - 11:41 pm
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you have to be careful with the bigends because by the time they make a noticeable movement bits of the rollers are starting to come apart and make their way up into the cylinder and head which can wreck both

D? - D10- D14 Bantams 350 AJS -500 Triumph http://bsanotru.....lfire.com/

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Nick_S
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January 4, 2020 - 12:20 am
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If I may just add my input to this. I'd personally have got the big end sorted first, as has already been said by others, it will only get worse & probably quite rapidly when the top end is done!

Anyhow, good luck with it & let us know how you get on.

Cheers,

Nick (another one)!

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nickjaxe
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January 4, 2020 - 11:21 am
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I agree with the big end comments above...you could wreck the engine if ignored.

If its been stored for a while...rust can easily get on the crankpin and rollers....soon as the engine is brought back to life the rust rubs away and you have rapidly wearing surfaces.

When my crank was stripped I think the clearance was under 10thou...but the re-builder said it was goosed...you could bearly feel it by hand.

My Bantam video              https://www.you.....jpOFmzRZRI

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JustinW
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January 4, 2020 - 11:30 am
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Ok thanks for all the advice. I'll get it sorted. I'll see how PJ get on with the rebore, and then ask if they'll do the crank. Their website says they specialise in 2 stroke cranks.

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nickjaxe
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January 4, 2020 - 11:34 am
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Let us know how you go on with PJ now one of our main engineers is out of action...well in the North West that is.

I would be interested to know if anybody has had a Bantam Barrel re-sleeved by them and how it worked out in practice.

Nick.

My Bantam video              https://www.you.....jpOFmzRZRI

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JustinW
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February 15, 2020 - 6:56 pm
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Ok an update.

PJ did a great job reboring +20 and supplied Rex Caunt piston and rings. Bore honed and ports radiused. £95 inc postage. I refitted barrel using big zip ties to hold down rings..not sure if that's a thing but worked a treat. They slipped down out of the way and I cut them off.

Alpha bearings (alpha-bearings.com) did the full monty on big end. Ground conrod to accept bespoke liners, new crankpin, new small end reamed to size. Refitted the discs which were spinners. £300. 

Lot of dosh, but now engine is totally sorted with fresh seals too. Both companies a pleasure to deal with.

All put back together and the compression is instantly noticable as improved.

Fired up and sounding different too, super smooth. I was extremely nervous that I'd put all the bits back together correctly. I would have been super peed off to have to take it all apart again!

Haven't been for a spin yet due to utter filthy state of lanes, but super excited to try her out.

Anyway, it's great that there are still firms sorting our old bikes engines out.

Thanks for the advice which got me to where I am. My crank seals were rock hard, the big end was gone, and the piston was sloppy... amazing it ran at all really.

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NeilB
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February 15, 2020 - 7:34 pm
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Good to hear it's all back together and ready for another 50 years, and I think everyone has that same feeling with a rebuilt engine no matter how many they've done before, I know I do!

Don't forget the extra oil for running in, and take it easy for a few warm - up / cool down cycles thumbs-up

When I bought my bantam it didn't run, but a bit of wiring and servicing and it was soon brought back to life although it sounded like a puckle gun! The big and play and piston ring gap were appalling, as you say with yours, surprised it ran at all! 

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