Piston up grade
- Martin.S
- Posts: 443
- Joined: Sun Mar 18, 2018 12:02 pm
- Location: Bristol UK
Piston up grade
The combustion shape/dimensions appear to be virtually the same as my G5 head.
As the bore is the same and the deck height is within 0.029" (according to Don Maddens piston identification drawings) of the G5 piston I see no reason why the 650 piston will not fit the 350 lightweights.
Of course the stroke is longer on the 350 so the compression ratio would need to be adjusted perhaps by skimming the piston, thicker copper head gaskets and or thicker base gaskets - I was thinking 9.5:1 would be optimum.
Proof they say is in eating the pudding so I'm on the lookout to beg, borrow or buy very cheaply a scruffy 650csr piston to see how it fits and measure the compression.
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- clanger9
- Posts: 1163
- Joined: Sat Jun 20, 2015 9:38 am
- Location: Chester, UK
Re: Piston up grade
Apparently a 1959 650 piston will fit (with some modification). It will give a useful increase in compression.
http://www.jampot.com/forum/viewtopic.p ... 10#p135761
http://www.jampot.com/forum/viewtopic.p ... 10#p135761
1989 Moto Morini Dart 350
1993 Ducati 750SS
1993 Ducati 750SS
- Martin.S
- Posts: 443
- Joined: Sun Mar 18, 2018 12:02 pm
- Location: Bristol UK
Re: Piston up grade
Yeh thanks Paul, I have read about using a '59 piston but the compression ratio is unknown and have you seen the shape of the piston?
I was thinking of seeking out a 1960 onwards 650 CSR piston with a flat top that would allow a faster flame path(apparently) or building up the combustion chamber with weld for a more modern shape but advice is needed here. But if anyone has a '59 or 1960 onward piston to donate, lend or sell very cheaply for compression measurements it would be most appreciated.
Cheers
I was thinking of seeking out a 1960 onwards 650 CSR piston with a flat top that would allow a faster flame path(apparently) or building up the combustion chamber with weld for a more modern shape but advice is needed here. But if anyone has a '59 or 1960 onward piston to donate, lend or sell very cheaply for compression measurements it would be most appreciated.
![Wink ;)](./images/smilies/icon_e_wink.gif)
Cheers
- clanger9
- Posts: 1163
- Joined: Sat Jun 20, 2015 9:38 am
- Location: Chester, UK
Re: Piston up grade
I see what you're thinking. I don't know of anyone managing to get 9.5:1 on a 350 engine (it's more like 8:1 with the 650 piston), but that would be a useful goal, especially if you can get there with a flat top piston. Tricky, given the deeply recessed combustion chamber...
1989 Moto Morini Dart 350
1993 Ducati 750SS
1993 Ducati 750SS
- Martin.S
- Posts: 443
- Joined: Sun Mar 18, 2018 12:02 pm
- Location: Bristol UK
Re: Piston up grade
I've bought myself a nice used post 1960 650cc piston. The gudgeon pin is the same size as the G5. The outer diamter of the crown contacts the head so would need reprofiling. I've taken measurement with some thin oil and I recon that it's about 9:1.
The only problem is that the valve cut outs are in the wrong place. Some people on t'net have welded up holes in two stroke pistons with success but many poo poo the idea. So I'm going to try and get some professional advice here. I could possibly use Lumiweld but not sure if it would stand the heat - how hot does a piston get?
The only problem is that the valve cut outs are in the wrong place. Some people on t'net have welded up holes in two stroke pistons with success but many poo poo the idea. So I'm going to try and get some professional advice here. I could possibly use Lumiweld but not sure if it would stand the heat - how hot does a piston get?
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Re: Piston up grade
Hi
Assuming the non standard piston is on the right:
I'd be very wary of removing too much material, but if you're only creating a clearance then maybe OK.
I certainly wouldn't try to add material and especially not Lumiweld - better to sacrifice a little compression than the engine when the bond breaks.
Regards Mick
Assuming the non standard piston is on the right:
I'd be very wary of removing too much material, but if you're only creating a clearance then maybe OK.
I certainly wouldn't try to add material and especially not Lumiweld - better to sacrifice a little compression than the engine when the bond breaks.
Regards Mick
- Martin.S
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- Joined: Sun Mar 18, 2018 12:02 pm
- Location: Bristol UK
Re: Piston up grade
I made up a mock piston to see how much i'd have to remove and practice machining the valve pockets - not much came off to clear the head.
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- Martin.S
- Posts: 443
- Joined: Sun Mar 18, 2018 12:02 pm
- Location: Bristol UK
Re: Piston up grade
Well I went over to see Steve at Piston Broke here in Bristol. A definite no no to welding pistons - he tried it way back when and had troubles with distortion and expansion problems developed. He didn't think much about changing the combustion chamber in the head either - the valve seats aren't concentric with guides - a lot of work to put them back in - Oh bum!
So maybe take out what's needed for the valves on the 1960 650 piston and as Mick said put up a little less compression with the cut outs that are already there or try the 1959 650 domed piston - anyone got one to try?
So maybe take out what's needed for the valves on the 1960 650 piston and as Mick said put up a little less compression with the cut outs that are already there or try the 1959 650 domed piston - anyone got one to try?
- ajscomboman
- Member
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- Location: HAMPSHIRE UK
Re: Piston up grade
I have a +20 that has been done. What bore size are you looking for STD i suppose? I have a few but not sure if I have any single ones only pairs. I'll have a look.Martin.S wrote:Well I went over to see Steve at Piston Broke here in Bristol. A definite no no to welding pistons - he tried it way back when and had troubles with distortion and expansion problems developed. He didn't think much about changing the combustion chamber in the head either - the valve seats aren't concentric with guides - a lot of work to put them back in - Oh bum!
So maybe take out what's needed for the valves on the 1960 650 piston and as Mick said put up a little less compression with the cut outs that are already there or try the 1959 650 domed piston - anyone got one to try?
I hate to say, I told you so, but, I told you it wasn't worth trying to reinvent the wheel! I went through the same process 20 yrs ago and came to the conclusion a 59 piston was the easiest way to go as it doesn't require cutouts for valves etc and no other machining other than the skirt to clear the flywheels.
I do like the trial piston you knocked up though
- Martin.S
- Posts: 443
- Joined: Sun Mar 18, 2018 12:02 pm
- Location: Bristol UK
Re: Piston up grade
My barrel is +.030". So whatever piston I end up using will have to be +.040". But for a trial fit anything that'll fit in the present bore as I want to measure the compression ratio.
I suppose I could alter my mock up piston to replocate a '59 one if the radius of the dome were known.
Cheers
I suppose I could alter my mock up piston to replocate a '59 one if the radius of the dome were known.
Cheers