LCGB Forums

I have an oversized GT186 barrel,
it uses a ProX 31K 64.5mm piston,
at BDC the top ring peg is exposed in the boost port,
is this normal for GT186 kits?
The peg is not in the centre of the port and I don't like the idea of using it like this.
Top ring when checked had a massive ring gap.
Ports look virgin,untuned

Comparing it (in a Bike store by eye) with another Wiseco YPVS RD350LC piston,the ring pegs were almost in the same position,
maybe 1mm or less closer,it wouldn't solve my problem.
What piston does the GT186 come with,Mitaka?
I've seen Mitaka pistons for RD350LC in two types,4LO and 31K,
they have the ring pegs in different position (3mm difference).

The distance of the pegs in my piston is 30mm
Pic with the piston at BDC and the piston

Another interesting question D.

I acknowledge your dilemma. It is very difficult gauge the position of the piston ring peg without putting the piston on a crank and putting a barrel on top. Sometimes this can be really awkward if the bore isn't (yet) big enough for the piston. However, I have found considerable differences in apparently identical pistons, such as Suzuki TS185, albeit from different manufacturers. Ring peg pin positions can vary by several millimetres.

I don't use the GT kit (perhaps I should), but I do use RD pistons in my SR185 reed-barrel. I have found that the YPVS piston is better than the old RD piston, with better windows in the skirt. Perhaps you should consider this piston as an option. It may have the ring peg in a position that suits your port layout.

Good luck with your quest.

Yes the Gt186 is a Mitaka Rd350lc/ ypvs
(4LO/31k)
Size 64 MM

When i had my GT 186 kit i always found 'yambits' cheap and helpful and bought over sized pistons for it up to 65.5.

Thank you for the input very good sirs
I've ordered a Mitaka 4LO 64.75mm piston,
I'll update in the future,
my aim if the peg is exposed, will be to open symmetrical both boost ports so that, the top peg will be in the port centre and bottom will still be covered.
Stay tuned...

This may be useful to know:

Grampian Motors have lots of information about the products they sell on-line, notably pistons.

They list both RD350LC/YPVS & RD350/400 in which the only notable difference in dimensioning is that of the “ring peg location” with them stated as “15.00” & “13.00” respectively

Thanks,
I've ordered the 4LO with 12.00 ring peg position.

Ι'll probably get the Mitaka piston,on my hands, next week.

I measured the ProX 31K,the pegs are 55 degrees apart,
in a 64.5mm piston, this gives an arc length of 30.96mm and a chord length of 29.78mm (rounded up)
Ideally I would like something close to 60 degrees or alternatively around 49 degrees apart (top peg in the mid of the boost port).

Post by Dimitrios_231 » Thu Oct 06, 2016 4:13 pm

Ι'll probably get the Mitaka piston,on my hands, next week.

I measured the ProX 31K,the pegs are 55 degrees apart,
in a 64.5mm piston, this gives an arc length of 30.96mm and a chord length of 29.78mm (rounded up)
Ideally I would like something close to 60 degrees or alternatively around 49 degrees apart (top peg in the mid of the boost port).

Hi Dimitrios.

You know the options as far as RD/LC pistons & the nearest I know of would be TS 185 but that has peg positions of 14.5 mm & is 3 mm less crown height. Depends if you have a base packer I suppose, or a choice of con-rod lengths.

Re-pegging is the other option but everybody seems loathe to do so. I’ve personally never had any issues with doing so

Another option I could put you on to are single ring pistons, but I'd need to check any variations from what you have.

Thank you,
I won't use base and head gaskets,(threebond only),standard 58/107 crank with 1.15 squish and 22.5ml compressed volume.
(I just felt like I went for confession )
No plans to change the crank (a 60/110 sounds nice for the TS piston).

There is also a compatible Wiseco RT180 piston but has the same peg location.
Re-pegging is not an option,the guy who re-bores my cylinders tries to persuade me but I don't want to lose my sleep

I run my rapido 250 with a single ring,no compression problems etc,
this runs with a two rings Asso piston where the bottom peg is exposed (huge inlet), so the ring was removed.

I'll wait for the Mitaka,
meanwhile,
I need to use all the known thermodynamic theories to improve the flow inside the transfer ducts

Got the piston,
the peg distance is shorter,
they are around 47 degrees apart,
as you can see in the pics the peg is closer to the centre and will give better ring support.

360 degrees inlet

Crankcase ported.

Few moons earlier

Piston Mod

Extra Port

Post by Dimitrios_231 » Tue Nov 08, 2016 2:22 pm

Extra Port

Hi Dimitrios

Presumably, the intention of the additional port will be purely for the inlet port to ‘communicate’ more directly with the crankcase.

That’s interesting as it jogs my memory.

A very renowned tuner must have experimented with this concept some years ago, as I found several OEM barrels ditched for scrap outside his workshop, one of which had a similar modification to what I expect to see next performed on your barrel. Obviously, I couldn’t leave the barrels there....

It may have been in another post on this Forum, but you mentioned 360° inlet timing being achieved via windows in the piston inlet side, which I believe is only true if you acknowledge that the inlet remains partly obscured @ TDC. Being pedantic (!) only a ‘cut-out’ will achieve ‘full’ un-obscured 360° inlet timing with modifications limited to the piston. In any case, the tuner Robinson dispels any value to the worth of such a modification.

However, just to show that some of us are paying attention, with this latest modification, you will achieve a divergence in the inlet feed due to the piston modification being windows rather than a cut-out.

Have you researched this prior to making the modifications &, if so, what gains do you anticipate?

Τhis exta port is common on two stroke motorcycle engines,
as you said,it will connect the crankcase to the reed port.

The piston holes opened to get fuel entry below the piston for 360 degrees,
so simple
Near to BDC with standard holes,the fuel entry was blocked by the piston skirt.

I didn't want to cut the skirt,the holes were already too close to the bottom,
I didn't want to make the piston weaker,

Believe me,it will fly

Post by Dimitrios_231 » Tue Nov 08, 2016 3:25 pm

Τhis exta port is common on two stroke motorcycle engines,
as you said,it will connect the crankcase to the reed port.

The piston holes opened to get fuel entry below the piston for 360 degrees,
so simple
Near to BDC with standard holes,the fuel entry was blocked by the piston skirt.

I didn't want to cut the skirt,the holes were already too close to the bottom,
I didn't want to make the piston weaker,

Believe me,it will fly

It will be interesting to see if it does 'fly'....

I’m glad I haven’t thrown out all those odd (mostly) large block barrels, including a Autisa alloy cylinder that’s been hacked about presumably for a different inlet manifold....

Now that development appears to be progressing as far as the choices of retrofit reed valves manifolds, there might be opportunity to tame some wild inlet duration piston ported barrels.

Warkton Tornado No.1 wrote:I’m glad I haven’t thrown out all those odd (mostly) large block barrels, including a Autisa alloy cylinder that’s been hacked about presumably for a different inlet manifold....

Now that development appears to be progressing as far as the choices of retrofit reed valves manifolds, there might be opportunity to tame some wild inlet duration piston ported barrels.

Totally agree.

My problem is always time,I don't know if I'll have spare time to finish it soon.