P200 Top end replacement - cylinder locating dowel
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Lurker
Vespa 90, T5 (sodomized with a 200cc motor)
Joined: 05 Jan 2020
Posts: 3
Location: Wiltshire
Wed Apr 07, 2021 10:13 am quote
Hi everyone,

I have been rebuilding a P200 motor that was suffering from low compression. Upon pulling the head, I was greeted with a highly scored surface, which was likely the culprit. In favour of time, I decided to purchase a new barrel and piston and keep the old one for re-boring in the future.

The barrel arrived today, the ports weren't perfect but that didn't matter as I had planned to alter the timings a touch with my dremel to better match the Simonini expansion chamber anyhow.

My issue comes in that the small dowel on the bottom of the cylinder that locates the cylinder relative to the casings is missing. Can I run the cylinder without this dowel? If not, where can I buy a suitable dowel?

Thanks.
Ossessionato
73 & 74 Rally, 76 ET3, 80 P200, 06 PX150, 61 Ser 2, 65 Silver Special, 86 & 96 Elite 80s, 2015 HD Road Glide Special, 2011 Ural Tourist
Joined: 18 Jan 2012
Posts: 3943
Location: Oceanside, CA
Wed Apr 07, 2021 11:31 am quote
Yes you can run the cylinder without the dowel. The 4 studs will position the cylinder accordingly.
Molto Verboso
2007 Stella 225
Joined: 02 Nov 2019
Posts: 1673
Location: Rochester, Minnesota
Wed Apr 07, 2021 12:38 pm quote
Dowel, what dowel are you talking about? I have never seen one on my 150 motor or my 200, just the 4 cylinder studs.

Last edited by Christopher_55934 on Thu Apr 08, 2021 4:18 am; edited 1 time in total
Ossessionato
1979 P150X, 1983 P200E, 1988 T5, 1995 PX200E, 2011 Yamaha Fazer 600 S2
Joined: 02 Aug 2015
Posts: 2257
Location: Veria, Greece
Wed Apr 07, 2021 1:00 pm quote
This one...



Molto Verboso
2007 Stella 225
Joined: 02 Nov 2019
Posts: 1673
Location: Rochester, Minnesota
Wed Apr 07, 2021 4:07 pm quote
SaFiS wrote:
This one...
Neither of mine have that dowel. I cut my own shims by hand and have never seen those. I have seen that hole and wondered what it was for.
Style Maven
74 50s x3 78 P200 x2 84 Cosa PK50XL2 58 AllState 68 Sprint 80 50special '66(?) Super125
Joined: 22 Nov 2010
Posts: 8151
Location: seattle/athens
Thu Apr 08, 2021 1:17 am quote
It's normally there and clocks the cylinder to the crankcase more precisely than the studs and their clearanced holes thru the cylinder.
Molto Verboso
2007 Stella 225
Joined: 02 Nov 2019
Posts: 1673
Location: Rochester, Minnesota
Thu Apr 08, 2021 4:24 am quote
V oodoo wrote:
It's normally there and clocks the cylinder to the crankcase more precisely than the studs and their clearanced holes thru the cylinder.
Now I have to find a dowel to put in next time I pull my top end.

Anyone know where to find one, on Scootermercato website?
Lurker
Vespa 90, T5 (sodomized with a 200cc motor)
Joined: 05 Jan 2020
Posts: 3
Location: Wiltshire
Thu Apr 08, 2021 8:09 am quote
V oodoo wrote:
It's normally there and clocks the cylinder to the crankcase more precisely than the studs and their clearanced holes thru the cylinder.
I figured this was the case; could the malalignment cause any issues (such as the exhaust port being shorted by the transfers in the piston), or reduce performance (perhaps by blinding the boost port)?
Enthusiast
Joined: 05 Feb 2018
Posts: 59

Thu Apr 08, 2021 10:00 am quote
When i set up my Pinasco 225 a few years ago now, I used the screw off end of a spark plug which fits well and seems to have worked fine.

Ive wondered if not installing this is the thing that brings on the weak Pinasco piston problem thats been discussed.
Lurker
Vespa 90, T5 (sodomized with a 200cc motor)
Joined: 05 Jan 2020
Posts: 3
Location: Wiltshire
Thu Apr 08, 2021 1:17 pm quote
JanP wrote:
When i set up my Pinasco 225 a few years ago now, I used the screw off end of a spark plug which fits well and seems to have worked fine.

Ive wondered if not installing this is the thing that brings on the weak Pinasco piston problem thats been discussed.
Good idea, I'll use one off of a new plug that I'll be buying tomorrow.

I think that especially with an aftermarket exhaust which forces the cylinder anti-clockwise, it is very possible that this could contribute to the piston shattering, as the boost port area which is normally at around 1-2 o'clock would find its way closer to 12.

This would allow the skirt of the piston more freedom to migrate into the boost port area, which means that the piston would then have to encounter a sharp cylinder wall before being 'knocked' back into the bore.

Quite possibly leading to metal fatigue over a long period of time.
Style Maven
74 50s x3 78 P200 x2 84 Cosa PK50XL2 58 AllState 68 Sprint 80 50special '66(?) Super125
Joined: 22 Nov 2010
Posts: 8151
Location: seattle/athens
Thu Apr 08, 2021 9:54 pm quote
That sounds like a lot of rotation. If, for the sake of argument, say the stud hole has a half mm of clearance.
Then that would mean that without the dowel pin, it could be forced to rotate 1/4 mm off of nominal which doesn't seem like it would be enough to do all that. Do the math on the stud pattern diameter's circumference and I suppose you could figure out the degrees...

Any large industrial fastener shop should have metric dowel pins or try McMaster-Carr online or just Google. The dowel is hardened steel and much better for this than the hollow aluminum spark plugs end I'm afraid.
Member
Vespa T5 251, Lambretta GP200
Joined: 27 Jul 2019
Posts: 40
Location: Leeds, England
Fri Apr 09, 2021 1:27 pm quote
Its not needed on that barrel. It is used on high end multi port barrels to make sure the ring gap does not align with a port.
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