DR177 Alternative Piston.
Post Reply    Forum -> Not-So-Modern
Author Message
Hooked
PX125E
Joined: 06 Oct 2018
Posts: 136
Location: Gravesend, Kent, UK
Sat Sep 07, 2019 9:47 am quote
Hello.
I have. DR177 and the barrel is going to be skimmed 1.5mm off the top. The Piston isn't much and I’ve been advised to use a Grand sport replacement. They cost £120 in the UK. Soooo could I use a Malossi or Polini one instead?

If it’s a bit shorter between the crown and Gudgeon pin, I can compensate by skimming the extra off the Barrel. Or can I?
Molto Verboso
1979 P150X, 1983 P200E, 1987 T5, 1996 PX200E, 2011 Yamaha Fazer 600 S2
Joined: 02 Aug 2015
Posts: 1560
Location: Veria, Greece
Sat Sep 07, 2019 10:36 am quote
Are you going for a re-bore?? If yes, you can get the 63,5mm piston directly from GS. It will cost you around £85 including shipping. Unfortunately the 63mm piston isn’t available for both the Polini or the DR...

https://www.grand-sport.de/Grand-Sport-RACE-177-D-635mm
Hooked
PX125E
Joined: 06 Oct 2018
Posts: 136
Location: Gravesend, Kent, UK
Sat Sep 07, 2019 2:52 pm quote
Thank you for that. I assume a Hone or rebore will be necessary whatever Piston I use so the cost will be the same.

I had a seize with the original Piston. I have polished out the marks but I was advised the piston shouldn’t go past 7500 RPM. That’s wank, as My 125 showed 71 MPH at 7900 RPM. I don.t honestly know what to believe with regard to my speedo/ Rev counter. I tried a speedo App on my phone and it indicated I was 6 MPH faster. The brutal truth is according to online Calcs I should be doing 70 MPH at 8000 revs. My amazing SIP vape and speedo said 71 MPH at 7900 revs. (Not far out but I want to be honest ).
Ossessionato
1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
Joined: 26 Sep 2013
Posts: 4822
Location: So Cal
Sat Sep 07, 2019 6:54 pm quote
Quote:
I have polished out the marks but I was advised the piston shouldn’t go past 7500 RPM.
I’m curious about this. What fails over 7500 RPM and why?
Hooked
PX125E
Joined: 06 Oct 2018
Posts: 136
Location: Gravesend, Kent, UK
Sun Sep 08, 2019 3:55 am quote
SoCalGuy wrote:
Quote:
I have polished out the marks but I was advised the piston shouldn’t go past 7500 RPM.
I’m curious about this. What fails over 7500 RPM and why?
On another post I was discussing tuning the DR kit. Lots of good advice given including “don’t go above 7500 RPM with the original piston as the thick rings don’t like it. Fit a 21 tooth primary gear to keep the revs down”. (Sorry if I’ve misquoted Jack221 there).
Molto Verboso
One or two fun scoots....nothing too precious
Joined: 17 Jul 2013
Posts: 1139
Location: UK (South East)
Sun Sep 08, 2019 7:27 am quote
The high quality 63mm GS rings that I use on mine appear to be chamfered to reduce the thickness where the rings make contact with the bore, despite being the full thickness in the piston grooves. I've always run mine with the stock CDI that is on the 2011+ PX, which is supposedly restricted to 7000 RPM. That's why I went with taller gearing (23T clutch).
Banned
2:6
Joined: 11 Jan 2007
Posts: 7445
Location: San Francisco
Sun Sep 08, 2019 9:43 pm quote
using the piston acceleration figures from Gordon Jennings, the stock 2mm rings are safe to 7007, 1.5mm rings are safe to 8142, and 1mm rings are good to 10061. (with 57mm stroke, 105mm rod)

The issue with thicker rings is flutter above that speed.

I think the DR had 1.5mm rings, while the GS has 1.0
Molto Verboso
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 1419
Location: London UK
Sun Sep 08, 2019 11:43 pm quote
DR rings are 2mm. So, reduce that 7500rpm max to 7000rpm if you want to play by the 1973 rules.

https://ve-uk.com/product/piston-ring-63mm/
Molto Verboso
One or two fun scoots....nothing too precious
Joined: 17 Jul 2013
Posts: 1139
Location: UK (South East)
Mon Sep 09, 2019 12:09 am quote
Just checked on the Grand Sport rings for the stock DR (or Polini) piston. I was convinced they were chamfered to reduce the thickness in the bore, but it looks like they are also 2.0mm. Higher quality but presumably the same RPM limits.

https://www.grand-sport.de/Piston-rings-GRAND-SPORT-POLINI-177ccm-630-634-x-20mm-1pair-D-630mm
Molto Verboso
1979 P150X, 1983 P200E, 1987 T5, 1996 PX200E, 2011 Yamaha Fazer 600 S2
Joined: 02 Aug 2015
Posts: 1560
Location: Veria, Greece
Mon Sep 09, 2019 12:22 am quote
These are for the stock or their stockish replacement piston. Their “better” piston uses 1mm rings...

9FDF16B3-17AA-4F0F-B66C-9C088A36986B.jpeg

Molto Verboso
One or two fun scoots....nothing too precious
Joined: 17 Jul 2013
Posts: 1139
Location: UK (South East)
Mon Sep 09, 2019 1:09 am quote
SaFiS, yeah I decided against the GS piston and just went for better quality GS rings for the stock DR piston. The total cost of the GS piston plus the cost of the DR kit and a better head (Polini 177 in my case) comes to more than some of the aluminum/nicasil kits.

Perhaps it makes sense if you are machining and extensively porting a DR cylinder, but I don't suppose many are doing that, Sime66 being one possible exception.
Hooked
1984 PX(177)EFL
Joined: 14 Apr 2017
Posts: 140
Location: Cornwall UK
Mon Sep 09, 2019 1:37 am quote
As I was mentioned, but not with anything to add really;
When I first built my tuned DR it went to 9000rpm in 3rd, although I didn't break it I was warned against it for this reason. The Ø63mm GS piston has been out of stock for years, it briefly returned earlier in the year, and is out of stock again except for 1st o/s. I've spoken to GS and they say they're a small outfit and a manufacturing run is very expensive, so not likely to be back any time soon.
I've de-tuned a bit, and my DR now still revs to 8000rpm in 3rd if I push it on a bit, but I don't need to; I'm changing up at about 7800rpm – it's there for a very short time, and not every time, and when I'm in 4th I'm back under 7000rpm.
I think, maybe, from my experience not from a book, a brief spell over 7000rpm isn't such a problem. I do have a set of 2mm GS rings to fit next time (better material same thickness), or maybe I'll go G.S. 1st oversize if appropriate.
Note also, that here we're talking about revs for a 57mm crank and 105 con-rod, go 60 or 110 and the piston speed increases because it's a longer distance per revolution. Keep in mind too, whether you're talking about max piston speed or average speed; max speed is at about 76° past T/BDC. (I did some reading at the time, but I'm rusty on it now).
My new project is 1mm rings and probably 60mm crank, so I'll have different piston speeds/rpm and different limits.
Your Rally Here   Vespa Wasp Pin Badges   Scooter Parts Company
Post Reply    Forum -> Not-So-Modern
[ Time: 0.7537s ][ Queries: 25 (0.0981s) ][ Debug on ]