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Low compression D10s/4
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Colin1
Glasgow, Scotland UK
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February 5, 2020 - 3:21 pm
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I measured 30-40 psi. The engine is very hard to start and won't tick over. Also a fair bit of vibration. Removing the head I found it easy to wobble the piston in the bore. So I removed the barrel to examine the piston and bore.

The piston is +0.020. I measured the bore diameter carefully with a digital caliper. It measured 62.05 to 62.15mm. I measured the piston 61.66 to 61.81mm. Do these measurements seem approximately correct for my piston?

With the piston in the bore I measured with imperial feeler guages a clearance of .0075 all round the piston at the top of the bore and piston. .004 at the bottom of the piston and bore. This seems to match the workshop manual. 

Measuring the ring gap with imperial feeler gauges .044 top and .045 bottom. Middle of bore .044. So that would suggest minimal bore wear. There is no step at the top of the bore. I centred the ring each time with the piston.

The ring gap seems much too big compared to the workshop manual's .013 max. 

My questions. 1) Can I get away with fitting larger rings. 2) would it be better to replace the piston and rings. 3) should I rebore to +.040 with new piston and rings. 

Incidentally I think the big end bearing might be responsible for the vibration, there is some slight perceptible play to the touch. 

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Stoo63
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February 5, 2020 - 4:01 pm
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Hi Colin and welcome to the Forum. I'm in Firhill. There's a couple of us round Glasgow.

You will find just about everything you could ever want on the Forum via the FAQs and technical sections. Here, ** Please log in to view **.....ad-photos/

Have a good nose around the forum and enjoy your bike!

All the best

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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Colin1
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February 11, 2020 - 10:39 am
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I can't quit get to grips with my bore and piston measurements that all seem to met the 62mm and +.020 expectations for a good bore. However, as can be seen from my previous post the ring gap is too large by the same measurement top, middle and bottom of the barrel (.044). I purchased new rings +.020 but these were identical in measurement. 

Is there another solution, such as slightly larger rings to take up the gap or must I rebore to +.040 and fit a new piston and rings. At the moment this seems the only solution open to me. 

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AdrianS
Harlaston Staffs
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February 11, 2020 - 1:36 pm
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Is your measuring equipment very accurate? It may be worth taking the piston and barrel to an engineering works or properly equipped bike shop for measuring and an opinion. If new rings have still got too big a gap I feel the only way forward is a reborn with new piston and rings.

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mike p5xbx
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February 11, 2020 - 4:20 pm
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it can be very difficult getting accurate measurement from the barrel and piston
but if you new rings have too large a gap and you can wobble the piston fore and aft in the bore
and you have a low compression test as you said in the first post you probably need a rebore

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

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NeilB
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February 12, 2020 - 8:36 am
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When I got my bike it had a +020 piston and you could still see the honing lines in the bore, but the ring gap was way above the limits. Cue a trip to the local engine specialist (every machine in there looked like it was older than me) and we agreed that someone had clearly machined it to correct size and *then* honed it rather than slightly undersized and then hone it to size.

If the ring gap is too big, it needs to go up a size, but if you're getting banging too then I'd suggest checking your con rod for vertical movement on the big end bearing. 

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bantammad
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February 12, 2020 - 12:58 pm
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Hi bantammad ere just looking at your figures for your bore diameter 2.4468 to 2.4429 that would equate to a plus 0.040 piston also you have 0.0035 piston clearance for example take std bore 61mm = 2.4015 + 0.040 = 2.4415 + 0.0035 piston clearance so the target will be 2.445 on that basis I think you will have a + 0.040 inch bore hope I’m some help I’m a dinosaur Les

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Blue Heeler
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February 12, 2020 - 1:38 pm
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Hi Colin,

Don`t forget when measuring piston, that pistons are tapered, narrower at the top than they are at bottom of skirt, to accommodate for combustion heat up top. Skirt gets fresh 2-stroke mix directed at it from crankcases constantly.

Crankcase split required and crank overhaul too, from your description. Good luck.

Also that vibration is known to loosen flywheel cover plates...if fitted. I have a D-reg 3-speed D10, but not had it apart to check what flywheel covers if any it has, or how they are fitted. My D14s have riveted plates which did work loose.

With your barrel off, you should be able to rotate the crank to check the flywheel cover plates...but as you`re probably going to split the motor, make sure you check them then. Put some photos up if you can, I`d be interested to see.

Cheers,

Blue

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mike p5xbx
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February 12, 2020 - 2:41 pm
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bantammad said
for example take std bore 61mm = 2.4015 + 0.040 = 2.4415 + 0.0035 piston clearance so the target will be 2.445  

standard bore is 61.529mm = 2.4224
+20 would be…..62.037mm = 2.4424
+40 would be…..62.545mm = 2.4624

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

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bantammad
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February 12, 2020 - 2:45 pm
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Hi bantammad ere you’re right about that one Blue l have had a bit of nipping up on my 186cc six port barrel down to expansion found nipping up on each side of the exhaust port. On the over run when shutting down the throttle.cured that struck piston into the lathe and polished it midway to the crown now running like a it should.thats the special commended on some time back having fun Les 

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Blue Heeler
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February 12, 2020 - 3:03 pm
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Good work Les, I don`t have metalwork lathe skills, only did a bit at Tech.

I suppose we should mention to folks with running-in issues...there`s been a few recently... to remember to blip the throttle occasionally, eg especially downhill when coasting. It`s probably been brought up before on here....

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bantammad
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February 12, 2020 - 3:13 pm
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Hi bantammad ere yes Mike you’re 61.529 conversion to 2.4224 inch imperial are correct and so on how ever my example was for a bore of 61mm diameter etc happy days Les 

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bantammad
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February 12, 2020 - 4:20 pm
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Hi bantammad ere just a note of interest see piston fit and clearance Hepolite specifications or look at the search engine on the net but that will also depend on piston quality expansion rate and so on what an interesting subject wot have fun Les 

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cocorico
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February 12, 2020 - 9:01 pm
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Have you looked in FAQs? ** Please log in to view **

1956 D3 running, lights to sort. 7 other bikes in the Barn. 1950 D1 engine being rebuilt (slowly).

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bantammad
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February 12, 2020 - 9:57 pm
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Hi bantammad ere there’s many piston to bore running clearance specifications it mainly all depends on the make of piston and expansion of piston we used a mobile boreing bar which was fitted to the engine block in situ those were the days I’m a dinosaur regards Les

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bantammad
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February 13, 2020 - 12:53 am
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Hi bantammad ere yes cocorico I’ll go along with that but take another look at Colins description only 30to40 psi piston sizes to bore size plus his ring gap with what his + 0.020 marked piston looks to me like it’ should have a + 040 piston happy days Les 

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Ringting
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February 14, 2020 - 8:41 pm
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Hi Colin,

According to my manual the standard bore size is 61.5 mm. A 20 thou (0.020") rebore should give a bore size of 62.08 mm. The bore must be round, ie measure the same back to front and side to side. At 62.05 by 62.15 you seem to have 0.1 mm of ovality which is way too much. 

Your piston measurement of 61.81 was, I assume taken at the bottom of the skirt, front to back, (ie across the pin axis). (Pistons are made tapered and oval to allow for expansion).

Bore diameter, minus,  piston diameter = piston clearance.  62.05 minus 61.81 = 0.24 mm  skirt clearance. That's nearly 10 thou, and should only be 4 thou. 

When measuring skirt clearance with a feeler gauge bear in mind that the radius of such a small bore is quite tight and a feeler gauge isn't very flexible. Better to make up the size with 2 or more thinner more flexible blades. Just make sure they are very clean.

If you are not absolutely sure of your measurements then best get a machine shop to check for you. 

If you do need a rebore check on here for recommendations for 2 stroke specialists. There are some horror stories. Beware!

Alan. 

D1, D14/4, Guzzi LeMans, Triumph Trident 900, Maserati 160 t4. Mk1 Mini Traveller, Berkeley T60.

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