Engine overhaul Yamaha DT175

[LanaTheGreat]

I recently stripped my wife's Yamaha DT175 engine and gearbox. The cylinder/piston/rings etc must be replaced as it is top oversize, so there goes my bank balance. The other problem I saw was that the gear mesh has pit marks.

Is it possible that these pit marks in the gear mesh can be machined out, or do I have no other option than to replace all the gears?

[sumpgard]

hi you will have to buy new gears machining them is not possible try ebay or a good breakers i have repaired gear boxes how bad are they is the gear box noisy just change opposite gears. your cylinder you could always get it resleaved try pj motorcycle engineers in wolverhampton

[LanaTheGreat]

Thanks for the reply. Unfortunately I am in South Africa so that will be difficult for me to try that place. Only sixth gear is a bit noisy, the other gears sound fine. I doubt whether it will be possible to resleeve the cylinder since it is a 2-stroke engine and has all those ports in the sleeve, engineers would probably laugh at me if I do ask them to do it, since they mostly do work on car engines.

What do you mean when you say I should try the opposite gears?

[sumpgard]

when you replace a worn gear always change the worn one also the ones the bad gear comes in contact with.

[LanaTheGreat]

I went to Perry Yamaha in Durban, South Africa. The workshop guy took a good look at the gears, and said that they are okay

The nut holding the gear shaft was loose, hence the noise that came from 6th gear, and he showed me the slight damage to the shaft, but said everything was fine.

[sumpgard]

thats good news i have just rebuilt a ktm gear box parts cost £240 thats with a trade discount.

[Pete.]

Gears can suffer a bit of pitting especially on low-powered bikes like that one and still be ok to use. They will likely last quite some time. The faces of gears don't slide against each other much as many people think - they actually just press against each other and the contact-faces 'roll' together and apart, only siding at the start and the end of meshing. Personally, I'd keep using it until it started making horrible gearbox noises and keep my eye out in the meantime for a spare-parts engine.

https://www.mech.uwa.edu.au/DANotes/gears/meshing/gearAnimation.gif
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[hmmmnz]

there is a good company here in nz that replate bores,
they will even replate on old cast iron ones,
so i dare say there should be someone in s.a that also would do it,
failing that send them over here, its about $400nz to get done
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the humans are dead
I kick arse for the lord
Wiring Diagrams BIDNIP it bitches

[Kickstart]

[LanaTheGreat]

I will find out from the engineering places here in Durban, there is a whole bunch of them close to where I live.

Does the non-Yamaha mean that it can be a motor car piston, or must it be for a bike?

[Kickstart]

Hi

Need to be a suitable 2 stroke piston (which pretty much limits it to bikes).

A quick look and it seems that Wiseco do replacement pistons for the DT175 at up to 2mm over size (wouldn't surprise me if the max oversize from Yamaha is 0.5mm or 1mm oversize).

https://www.wiseco.com/Catalogs/Powersports/YamahaCycleDirt.pdf

All the best

Keith
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[LanaTheGreat]

Thanks for the replies. I had a look at the catalog and the DT175's mentioned there are for the old types (1979-1981). Those models look way different from the 1994 model that my wife has.

Furthermore the pistons that Wiseco sell seem more expensive than the ones I get from Yamaha. Also Keith, you were right, Yamaha offers only oversized pistons to 0.5mm.

[Kickstart]

Hi

Didn't know they did the DT175 that recently, but just checking the Yamaha South Africa web site it is a current model. Still appears to be using the old air cooled engine (bore and stroke are the same as the late 1970s ones).

I would say there is a fair chance the pistons are interchangeable.

Wiseco pistons are generally very high quality forged items. Probably rather better quality than the stock Yamaha ones.

For something cheaper a quick google found these people more local to you, with over size pistons up to 2mm oversize:-

https://www.imoto.co.za/pistons.htm

All the best

Keith
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[LanaTheGreat]

Keith, thanks a lot for that. I dropped them an email and they said a piston kit will cost ZAR290.00, that is equivalent to US$32.00, a fair price indeed.

I quote the guy's reply: "The sleeve can be rebored, 66,5mm is first oversize (0,5mm), so you still have three rebores left there. The cost for the piston kit is R290-00 plus R50-00 for overnight to your nearest Post office. I must just check to see what sizes are available."

Funny he says that the 66.5mm is first oversize, as according to Yamaha, it is top oversize. I am not complaining. Do you think that this will be a good option to go for, and will the piston be of an acceptable standard?

I went to several engineering places today, there are loads of them in my area, yet none could help me with reboring the 2 stroke cylinder. I will keep looking...

[Kickstart]

Hi

I have no idea who makes the piston, so the quality might be fine or might be terrible. Similarly the rings. I know that a local 2 stroke place to me with a good reputation uses Mitaka pistons but changes the rings.

Should be pretty easy to get someone to rebore it.

All the best

Keith
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[LanaTheGreat]

Thanks. I can get a rebore for ZAR450.00 at a local bike shop. The pistons are in stock and I will order them on 15/01/2009.

Is it REALLY necessary to replace all the gaskets, or can I use Q-bond instead of using the original gaskets, especially the crankcase cover gaskets?

[Kickstart]

Hi

Would expect a full pattern gasket set to be fairly cheap.

You might get away with reusing the clutch cover gasket, but I wouldn't be certain. On the other hand it should be easy to do at a later date if it leaks. But it only takes one leaking gasket for you to have to strip down the engine again and have to replace the gaskets you did buy new.

All the best

Keith
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[LanaTheGreat]

The Yamaha bike shop says they only supply the gaskets as separate pieces, and not as a set, that suits me okay.

I ordered the piston, and it will be here on Monday, then I can at last start fixing the bike.

[Kickstart]

Hi

Suspect Yamaha do only sell the gaskets individually, but I would be surprised if you cannot get a decent quality pattern complete set. Found them listed in the UK (at ~£10).

All the best

Keith
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[LanaTheGreat]

I received the Emgo piston kit this morning (piston, gudgeon pin, circlips and rings). I purchased the small end bearing as well just to be safe. Also ordered the clutch side crankcase cover gasket, and the reed valve gasket.

I handed the piston to the engineer as he wanted it before he would re bore, and it should be done tomorrow.

The bike shop owner told me I just have to make sure that the port edges in the cylinder are not sharp as it can catch the rings. Will check that too when it is done.

Will get the wife to start cleaning the engine parts in preparation for re-assembly.

[Kickstart]

[LanaTheGreat]

I received the cylinder this afternoon. The job looks well done, but the Yamaha salesman brought it to my attention that the ports on the cylinder must be chamfered so that the rings don't stick onto it in operation and break. The ports were not chamfered as expected, and my friend let the engineer do it.

Some of the ports still do feel sharp, is it possible that I can chamfer them myself, using fine files?

[Kickstart]

Hi

Yes, do you can do it with a file. But don't go mad.

All the best

Keith
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[chillpill]

sorry to spam but how would you go about finding out if a different piston would fit ?

obviously theres the actuall size of the piston and the gudgeon pin size . but what else do you need to look out for ? i know 2 stroke pistons have ports in them would it be possible to cut ports into a piston ?

cheers

[Kickstart]

Hi

Points you mentioned, plus the offset of the gudgeon pin from the centre, height of the piston (above and below the gudgeon pin), type of ring, number of rings, possibly where the ring end gaps are (you might want to avoid these going over ports), weight, material, manufacturing method and cutouts for ports (which not all 2 strokes will have). There are loads of factors but many of them are unlikely to be an issue (ie, the offset the of the gudgeon pin is not likely to vary much), or are chosen for slightly different uses (eg, forged pistons).

Also things like the shape of the piston crown, although this is more of an issue with 4 strokes which have cutouts for the valves.

All the best

Keith
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