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Which Piston Matches Cylinder for Which Year?

  • Hawkboy
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Hi, new thread here from a new member in New Zealand. At least I think this is new a new thread because I couldn’t find if this had been addressed before. This is a bit long, but the background I think is relevant.

For a very small fee, I got a slew of DT175 parts; one almost complete bike, and two others stripped down to frames all jumbled up in several boxes. The three bikes have matching frame and engine numbers, but the some of the engine bits don’t appear to match up, more of this later. For our purposes, all three bikes are “MX” standard as in mono shock with provision for lights, indicators and ability to be road registered. The frame/engine numbers are:

3H8-000815 this should be a 1979 DT175F MX
3U5-062943 this should be a 1981 DT175G MX
3T6-050275 this should be a 1980 DT125H MX

Major parts in the boxes included two 175 cylinders marked 2A700 171cc, with accompanying heads marked as 2A7. One cylinder appears to have been ported with a raised exhaust port and enlarged transfer ports. There are three pistons, two of which are brand new (probably aftermarket), but marked as 1.00 oversize, with another used one marked 0.75 oversize. Also included was a 125 piston, cylinder (marked as 2A600 123cc) and head. A couple of exhausts were included along with a handmade expansion chamber which probably matches the ported head, but unsure at the moment.

The 125 piston, cylinder, head and expansion chamber came installed on the frame/crankcase of the 175F. I got it started, but it was barely running, revving up and then dying which I diagnosed as a bit of an air leak, but was very surprised by some real shade-tree/back 40 maintenance I found when I took the head and cylinder off.

The installed head gasket for the 125 four stud cylinder had been adapted from a six stud 175 gasket. A couple of extra holes had been drilled in the face of the gasket and of course the 175 gasket was bigger than the bore of the 125, but it had managed to clamp down and seal for some compression. It didn’t look good for what I might find at the base gasket.[img]file:///C:/Users/c19038/AppData/Local/Temp/msohtmlclip1/01/clip_image002.jpg[/img]

My lack of confidence with what I would find at the base gasket was well deserved. The gasket was from another similar engine, it didn’t fit the 125 cylinder, and also didn’t fit the 175 cylinder. The gasket material to seal outside of the transfer ports was larger than the flange and this was the source of the air leak, well not a leak, more like an open barn door.[img]file:///C:/Users/c19038/AppData/Local/Temp/msohtmlclip1/01/clip_image004.jpg[/img]

To try things out, I installed a correct base and head gasket with a standard exhaust/muffler and it started second kick, idled ok and revved up nicely. Result!! At the very least from the boxes of bits I had something that would run.

Now we get to the interesting part of the 175 engines.

I dry fitted one of the 175 cylinders with one of the new pistons onto a crankcase and found at TDC the piston extended beyond the bore by approx 5mm, this was the same for any combination of the cylinders, pistons on each of the crankcases, but interestingly the 125 combo came to the correct piston cylinder height on the same crankcases.

The stroke on all the crankcases is the same at 50mm as it should be, which leads me to believe the pistons are incorrect and don’t match the cylinders, or is it that the cylinders don’t match the crankcases? The 175 pistons have a measurement from the crown to the centre of the wrist pin of 35mm (can’t measure the 125 piston as it is still installed). If this measurement is reduced by approx 5mm so the crown doesn’t top out, it seems it might pooch all the other port timing.

Leaving aside any oversize/rebore issues for the moment as I’ll probably go a size up as matter of course, some other research which may be leading me down the wrong path, shows that a manufacturing code of 2A7 on the cylinders might mean they are from a 1978 175E, but I’m not sure if the codes read across like this between engines to cylinders. Also, how much different can it be if the stroke is the same although the engines might be different enough from the earlier four long thru’ studs straight finned head to the six stud radial fin head? My cylinders have the recessed cylinder mounting only found on the later models.

All three cylinders I have, including the 125, have the same cylinder length 117.5mm, with the same base to skirt measurement of 37mm, so on the face of it, if I had the correct piston it should be ok. The standard piston for a 1981 175G should be (I think) 3J1-11631-00, with oversizes up to -04 for piston/ring sets, but I can’t confirm the height of the piston from the crown to the centre of the wrist pin.

There does seem to be a myriad of slight differences on the motors over the years, but the pistons may have come from a 175 Ag version or a TY175, or even a later model up to recent manufacture. I don’t know why the previous owner did what he did and I don’t have the history of what was ordered or why.

The question is which piston(s) do I need? Any help for what I should order would be most appreciated.
Last edit: 17 Jan 2022 14:48 by Hawkboy. Reason: Formatting.
17 Jan 2022 14:43 #1

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Wonder it they changed the rod length at some stage through the years of those models. Or is it an IT175 piston or something like that--if they changed gudgeon height.. Seeing you have the piston & it might be better to make a base plate to lift it up 5 or 6mm. Think everything yamaha do seem to go in 6mm steps. I'm using a TY175 piston & cylinder on my AT1. AT1 is piston port 125 so due to gudgeon height change i had to make a base plate--a job i hate without proper gear lol. Anyway--welcome. I fell asleep halfway down the story sorry but looks like you have more than enough to keep you busy. Think you said one combo would barely run, but is the piston coming to the top on that one--125 was it. I'll catch up & re-read later. Are you down south in "The Main Land". By the way i see a few non links to pics in your story?, were they meant to go somewhere. Cheers from a North Islander.
17 Jan 2022 15:12 #2

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Similar thing has been discussed here before.  Get Yamaha pistons for the 78-81 DT175 and you will be fine.

I believe the 175 pistons you have are being sold on ebay (and perhaps elsewhere) and fit Asia market 1982-on models... pin height is wrong.  

Lots of aftermarket can be used but stay away from cheap Chinese pistons as they tend to self-destruct.  Yamaha Blaster piston of suitable size can be used but shouldn't be difficult to find a correct piston.
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
17 Jan 2022 16:04 #3

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Replied by Hawkboy on topic Which Piston Matches Cylinder for Which Year?

Thanks for the replies. I'll just have to find some NOS oversize pistons. On the north island outside Palmy. Sorry for being a bit boring, no pun intended. The tale should have been broken up with a couple of photos, but haven't yet worked out how to include them. I'll keep trying as they are doozies.
17 Jan 2022 17:03 #4

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Maybe MarkT will chime in he’s the expert
17 Jan 2022 17:31 #5

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Here's one of the threads to add to my post above...  was about a cheap 125 cylinder kit but two or three people discuss the same issue you're having

yamaha-enduros.com/index.php/forums/gene...wtfff-is-this#141134
 
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
17 Jan 2022 19:08 #6

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Not a boring story--just me with too many head injuries & fall off the perch if i have to concentrate for too long. haha. Look forward to pics. Just save to ya puter & use the "reply" button not the "quick reply" Then "atachment" then "add file" then "insert" then "submit". If you don't use 'submit' the pic will stay small but we only need to click it to expand so no biggy.
18 Jan 2022 15:35 #7

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Replied by Hawkboy on topic Which Piston Matches Cylinder for Which Year?

As promised some photo's. Base gasket with gap around flange is similar to others, but this was installed cylinder tightened down and the bike eventually ran, very badly, but it ran. The same with the head gasket. I think it shows just how bullet proof these engines are. When I got the 125 running with a a couple of new gaskets, the chain was a bit noisy and I found this    high quality welding job.

Has anyone else seen some real corkers that shouldn't have worked, but did?
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20 Jan 2022 16:38 #8

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Replied by Tinkicker on topic Which Piston Matches Cylinder for Which Year?

To add to Mark Ts post about piston quality.  Buying aftermarket pistons can be a nightmare.  MarkT is correct, you have a later piston.  You need a quality 78 to 81 piston.
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 ART made the OEM Yamaha piston.

Of course, there will probably be a Wiseco offering but pricey.

Then you have the Japanese made Mitaka, ProX and TKRJ offerings.  As far as I am aware, these three are produced by the same manufacturer in the same mould and the only difference between them is when they are making a batch for say TKRJ,  they just change the nameplate in the mould.

Also, as far as I am aware, the supplied rings are made by Riken.

I can only speak from experience with TKRJ branded pistons.  Back in the day I was Senior Technician at a very large, mainly off road motorcycle project with a fleet of some 75 off road bikes and quads ( PWs, RTs, TYs, TTRs, CRFs LTs Scorpas, Gas Gas,  Montesa, and Sherco).  these were hired to and thrashed to within an inch of their lives by the general public 7 days a week. When it came to aftermarket Yamaha two stroke pistons, our wholesale trade parts supplier stocked TKR pistons (it was their sales rep who told me about the moulds).

Despite typically being thrashed mercilessly off road with engine cooling fins clogged with mud and overheating before being plunged into a cold stream, over many many hours of use, and in 12 years, I never saw a TKR piston crack or break up.  Typically TBO of an engine at the project would be around 11 months or 1200 hours.   That's a pretty good testament.

Of course there will always be purists who appear out of the woodwork who say they have had bad experiences and would only fit wiseco or genuine or whatever,  I can only give my own experience and had to work within tight budgetary constraints.  Even though we were sponsored by Yamaha, with heavily discounted trade prices, OEM cost vs benefit did not work out. 

I chose TKRJ when sourcing a piston for my 79, 175 engine rebuild, so I have put my money where my mouth is.  The piston arrived in a Mitaka box, but the piston and pin were branded TKRJ.

Of course I would go for OEM if the prices were close, but £120 vs £30 is a no brainer given my experiences.  Besides my DT will be treat with kid gloves, not thrashed.

For comparison, Chinese fleabay pistons typically cost around 15 - 20 quid and are best avoided.

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Last edit: 21 Jan 2022 00:18 by Tinkicker.
20 Jan 2022 23:10 #9

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Hey Tin Thanks for the great info sounds like you had fun
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21 Jan 2022 07:27 #10

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