IT175 pistons

[Graeme M]

I posed this over at the IT forum but not much interest. I have found that there seem to be varying arrangements for ring locating pegs on the pistons for the various models. Not being particularly knowledgeable around such things, I am wondering what these differences mean? I ask because I have two pistons - a Wiseco 374 and an NHK.

Now, the Wiseco uses the wider peg spacing, while the NHK a very narrow spacing. About 40 something mm versus about 28mm. Stock IT pistons seem to vary a bit - the earlier IW2 and 3R6 look to be wide, while the 5X8 is narrower (although not quite as narrow as the NHK).

Interestingly, stock DT175 pistons (3J1) seem to use a very narrow spacing like the NHK, but then again, not all of them because I have seen a photo of a 3J1 with wide spacing like the 5X8.

What the?

I guess my real question is - can the NHK piston with the narrow (28mm) spacing be used in the 1980 G model?

[Graeme M]

Here's some photos:

http://www.yamahait.com.au/forum/index.php/topic,17226.60.html

[GMC]

Bottom line is those ring pegs are supposed to stop the ring ends from wandering into a port where the ends may snag in its travels.
So if you place a piston in the barrel and slide it through its motions in the correct alignment and the pins don’t show in the ports (take the reeds out to see properly) then all should be fine.
If the pegs show in a port hole then I wouldn’t use it myself.

[tony27]

I read the whole thread Graeme & thought it was quite well explained, the intake port width is what governs where the pins are.
Seems the 82,83 pistons are different to the earlier models & not as common. Pro-x appears to be the only company that makes them for those 2 models

[sleepy]

It's more to do with tranfer and boost ports on the rear of the cylinder. The ring ends need to sit between any of the ports. Best to mark the ring end location on the top of the piston with a felt pen and slide the piston into the bore. Don't worry about the width of the intake port as the rings don't come down that far in use. If you want you can set the end gap width on a pair of dividers and see how it sits in the bore.

[Graeme M]

Ahh, that's what I was after. I had assumed that the rings don't come down into the intake port - I could of course assemble the cylinder with a piston and see how it looks, but as it's all apart I was hoping to establish the facts without bothering to do that.

It does seem to be transfer/boost related as the ring pegs don't completely miss the intake port for either configuration.

Thanks for the advice sleepy, I'll do as you suggest and check in relation to those transfers etc.

tony27, what threw me on the it forum thread is the suggestion that earlier 1W2 pistons have a narrower spacing - I think someone even showed a picture of an earlier piston which was marked as though it had narrow spacing.

As far as I can see, the 1W2 and 3R6 are identical with wide spacing, it's the 5X8 (ie 82/83) that seems to have a narrower spacing which suggests a change in transfer/boost configuration for the later models.

Anyways, I'll now check mine out with the spacings marked on the pistons and see how they line up - that'll tell which pistons can be used with my barrels.

[sleepy]

Wossner list the same piston for 76 to 81  IT175 but nothing after that in my catalogue. In my old wiseco catalogue I noted that the IT175 374 pistons had 47mm ring spacing with large windows and the DT piston 471 had 30mm ring gap and small windows. Pin height are the same on both those but the late DT175 had the pin 5mm further up the piston.

[Graeme M]

Thanks for that additional info. I will confirm when I get home tonight, but it looks like the 77-81 (1W2, 3R6) runs the pegs at 40-something mm (47?) and the 82-83 (5X8) at 38mm. The Wiseco 374 uses the 40-something spacing, whereas the NHK uses ~28mm spacing which seems to be, as far as I can tell, the same as the DT175 3J1. Has NHK based their piston on a DT175, and sell it as fitting DT and IT?

Anyways, based on all of that I am guessing my NHK won't work, but we will see.

[Graeme M]

I ended up concluding that the NHK piston is more than likely based upon a DT175 piston and is a risky proposition in any IT175. The aim of the 38mm and 40-something mm peg spacing is to keep the ring ends between the transfer/boost ports as noted earlier. The NHK's 28mm spacing puts them right inside the boost port.

I now have a Wiseco 374 on order.

[sleepy]

You can get away with the ring end across a narrow port but the preference is always a clean bit of cylinder. The NHK obyiously think it ok to cross the boost port and with only a small part of their sales being for IT's the care factor would be zero.
Wisceo or Wossner are a good choice, there probably is a Prox cast piston available if you wanted to keep it close to the original design.