Ring locator pin location

In the picture above you can see the pin locations marked on the top of the piston. IT piston has a wider "spread" between the pin gaps. If we look at an IT cylinder, you can see the area in the rear of the cylinder between the transfers where the ring end gaps are pinned... general area circled in red.



There's room to pin the rings pretty far away from each other...

Now here's an (ugly) 78-81 DT cylinder... note the transfers are crowded more towards the rear of the cylinder with a smaller circled area for the the ring gaps to be located.



If you used an IT piston in a DT, the ring end gaps would be pinned where the gaps would cross over a port which is BAD. That arrangement is likely to snag the end of a ring in a port and then bye-bye engine... often cylinder and possibly more is completely ruined.

So when swapping pistons from year to year, you want to check to make sure rings are pinned to go between the ports, not over them.

Other than that, the pin to crown distance is the same for all the 175's from 69-81... including the 77-79 IT175...

One more post...

So is a blaster piston a direct drop in replacement?

In your situation and under your restrictions of not wanting to do it right (bore it) I would try one of two options, both Wiseco.

Yamaha Blaster piston. Note that the Blaster piston, SKIRT MUST BE TRIMMED TO MATCH ORIGINAL PISTON In the pictures a few posts above, the skirt has already been trimmed to the correct height. Easy to do with a file or a machine shop can do it. Here's a pic of an untrimmed piston... almost all the "notch" will be gone... and both front and back skirts need to be trimmed.



Current Wiseco Blaster part number you would need: 573M06650

Again, check to make sure the ring pin locations will work with your porting!!!

Second would be the Wiseco 471M06650 for the 78-81 DT175.

Again, check to make sure the ring pin locations will work with your porting!!!

The above are mostly because you don't want to bore and in my experience Wiseco is a little larger than OEM... so your used bore with a likely smaller OEM piston would rattle like heck and not last long.

BEST OPTION

Send your cylinder to Bill Bune or someone good with an oversize OEM piston from Enduronut and have it bored and honed to correct factory clearance.

Using a new piston in your old bore with the damage you've described is likely to be false economy... money wasted. You'll just be re-doing it right again soon.

Mark, thanks for the info. I think we got a little off track. I did at one time train to be and make my living as full time motorcycle mechanic and then a main propulsion diesel mechanic in the navy. And now work as a technician in high speed process and automation. I fully understand piston ring indexing in piston port two stroke engines. My question was about if the crown to pin boss relation ever changed in the DT175 evolution. As I was getting a lot of conflicting information from sellers (including Wiseco) about year and fitment regardless of over size diameter. The only reason I’m not considering boring it now is because it the current bore mics (with a bore mic) to well within tolerance up and down the entire bore. If it had not I would be boring it to .75mm over. The damage to the piston was no doubt sustained from bouncing around in the bucket full of parts it was in when I bought the bike In pieces. Furthermore I only bought the “budget” piston because I couldn’t find a Wiseco for a 74-76 of ANY diameter oversized or not, while many places list them they show as sold out or discontinued when you try to order them. So now that I know there are more options other than just 74-76 piston I won’t have any trouble getting one of whatever bore diameter is necessary.

I thought I answered your question about crown to pin boss relationship back in post #2?

Then you were asking about differences... I went over some of them... most important being the ring end gap relation to the ports. (One thing I didn't mention was the rings drop below the ports so on a model (like IT175) with a large and high main intake port, you need to check that the gaps don't cross into the intake port.)

P.S. No offense intended. I didn't know your background and experience... and even if I did, it wouldn't change my answer. There are others reading this that might not have your knowledge.

(Raises hand) Like me! Having been a secondary and elementary teacher for 28 years, I always appreciate the "overkill" with which MarkT covers a subject. Assuming that the listeners know nothing, starting at the beginning, and politely and patiently covering the entire subject is the hallmark of a good great teacher.

I do appreciate all the info, just didn't want anyone to think I was trying to slap a new piston in a roached bore to save a few bucks. I know it is near impossible to judge folks skill levels on a forum (especially new members like myself). Moving on. So it looks like my options are either a 78-81 DT piston or a blaster piston. Are there any performance advantages (or disadvantages) to the blaster piston? it would appear the the blaster piston has substantially larger and higher ports, in my experience most changes of that nature to a two stroke trial/enduro bike only result in changing the power band to run more like an MX bike by robbing bottom end and increasing top end. I'm certainly not opposed to an overall increase in HP but not if it means sacrificing bottom end or drastically altering the powerband of a 45 year old trail bike that was not intended to be a WOT track flyer. Obviously the full potential of the blaster piston ports would not be fully utilized without some port work to the DT cylinder, witch i do intend to clean up the atrocious factory port castings but not really alter the geometry or port timing (but that's probably better covered in another thread).

The windows in the piston are not technically "ports".

Intake timing is ultimately controlled by the reed valve on your model. In my experience the larger holes in the piston will not rob bottom end for top end or appreciably change the power delivery characteristics in any way... though you might get a little more power with windows than without.

Thanks!

Just a side note... one of the later variations of the original 175 engine design was the water cooled DT200R street legal Enduro that came out around 1989 (not sold in USA) and the WR200 Enduro (not street legal)

It had case induction... reed valve opened directly into the crankcase so piston skirt never interferes with the intake flow.






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