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I'm about to embark on a new project. The aim of which is to build a cheap, nice local runabout using as much of my leftovers from previous jobs as possible.

I have a gp150 barrel and my plan for it is:

-Bore to 62mm and use asso piston, but maybe cheaper mahle is enough?
-raise transfers to something like 122 to 124 by using base packers only; no modifications to transfers themselves.
-raise exhaust to around 170 to 172 and widen at the top a little
-increase inlet to around 140

I'd like to do the port mods myself, just for the craic.

I'll need a 110 rod to mess with the transfer timings.

Possible drawbacks to this approach?

Thanks

have a go as long as you have the tools to do the porting work and a plan B if it goes tits up

missing lynx wrote:have a go as long as you have the tools to do the porting work and a plan B if it goes tits up

Plan b is li150 barrel I have
Plan c is admit defeat and get a grown up to tume gp125 barrel
Plan d is rt195

I was mostly wondering about the mahle piston and raising transfers using packers only though.

Its a grewat idea and will give you a better insight into the world of cylinder tuning. The only problem I can foresee is getting to shape of the exhaust port wrong. Don't be tempted to widen it much and you will need a trapezoid shape to stop the rings snagging.

I'd guess that to get the transfers where I want them I'll need to raise the barrel 1.5mm more than 3mm to compensate for the rod. This will almost certainly mean head mods, but I'll cross that bridge when I come to it.
For now, I'm wondering about the more fundamental issue of the bottom of the transfers being 1.5mm above the piston crown at bdc. Does this really matter?

dickie wrote:.............For now, I'm wondering about the more fundamental issue of the bottom of the transfers being 1.5mm above the piston crown at bdc. Does this really matter?

Not ideal, would a 115 rod give you more options?

coaster wrote:

dickie wrote:.............For now, I'm wondering about the more fundamental issue of the bottom of the transfers being 1.5mm above the piston crown at bdc. Does this really matter?

Not ideal, would a 115 rod give you more options?

No, i don't think it would, the problem is that I don't have the tools or skills to mess with tbe transfers. So the only way to raise them is with base packers. This is why they'd be above the piston crown at bdc. It leaves me wondering what is more important; flush with crown or height and its impact on blow down?

Today I managed to get an hour to myself so I had a fight with the motor I'm using as a basis (Indian GP125) and got the mag housing out (destroyed it) and the crank.

I reckon the crank is scrap as there is rust damage to the oil seal face. It's been cleaned up a bit with emery cloth, but there is still pitting that would need to be removed with a lathe realistically.

Am I being a fanny or should I just scrap this? Please base answers on the basis of the intent of this build which is a low spec, but well built 175; in other words, no more than maybe 15bhp (very optimistic) but very reliable.

When I see the photo it looks worse than it did in the garage.

20180902_151359 (2) by richardhenderson1968, on Flickr

Scrap that mate!

dscscotty wrote:Scrap that mate!

Anybody want to buy a gp crank? Needs a little work to the seal face.

OMG I was sure I was scrapping one that had pitting about 5% of that - think I'll hold fire

dickie wrote:Today I managed to get an hour to myself so I had a fight with the motor I'm using as a basis (Indian GP125) and got the mag housing out (destroyed it) and the crank.

I reckon the crank is scrap as there is rust damage to the oil seal face. It's been cleaned up a bit with emery cloth, but there is still pitting that would need to be removed with a lathe realistically.

Am I being a fanny or should I just scrap this? Please base answers on the basis of the intent of this build which is a low spec, but well built 175; in other words, no more than maybe 15bhp (very optimistic) but very reliable.

When I see the photo it looks worse than it did in the garage.

20180902_151359 (2) by richardhenderson1968, on Flickr

I'll buy the crank if it's going cheap

Then, to use it, I'd utilise a bearing with an integral oil seal (preferably 2RS) or find an undersized bore seal & modify the seal housing (if necessary) & clean up the crank seal face to suit, preferably in a lathe, though the 'old pillar drill @ very slow speed trick' might enable refurbishment with enough patience & care. As I personally don't give a toss about crankcase compression, the sealed bearing would do for me.

(Of course, if you think about the principle of the mag side bearing inner, you could get a sleeve knocked up by an engineering mate with access to a lathe & actually restore the crankshaft to the original size by shrinking it on.....)

not part of the tune obviously, but part of the enriching journey on the path to lambretta engine loveliness...

20180906_190411 by richardhenderson1968, on Flickr

cleaned up the heads, dremmeled off the punch marks then used an impact driver with a nice big fat bit on it. I even managed to rescue the screws to save a little bit more cash. Happy days.

FB_IMG_1536262242590 by richardhenderson1968, on Flickr

as I've never tuned a barrel before and I'll be getting it bored out another 5mm, I thought it would be prudent to practise putting a radius on the exhaust and transfer ports as any mistakes I make will be "rubbed out" when I get it re-bored. Anyway, It was easy enough to put a nice top edge on the exhaust but at least I've got a little confidence that I'm not going to totally wreck the barrel (probably, maybe) when I do it for real. It should be even easier when the barrel is 62mm. No photos as you can't see it; I just did it by feel.

started tuning in anger now. I decided to open out the transfers on their entry and along the length, but I won't be touching them in the barrel. I've read that this slows down gas entry into the barrel and it makes sense to me, so without the tools or know-how to get the roof angle etc correct, I'm leaving well alone. First two pictures shows how I marked them out and my rudimentary porting tools; I'm not doing it for a living so I'll just make do with this set up.

20180910_201126 by richardhenderson1968, on Flickr

20180910_201214 by richardhenderson1968, on Flickr

Half way through doing the first one
20180910_201141 by richardhenderson1968, on Flickr

One down, one to go
20180910_202819 by richardhenderson1968, on Flickr

Not exactly works of art, but the best I could manage
20180910_210647 by richardhenderson1968, on Flickr

I did the transfers first so that I can match them to the case before I fit the crank. That way the bearings aren't at risk of getting full of crap.

My theory is that maximum transfer volume gives you a larger charge ready to burst through into the barrel as the port opens. However, if you increase crankcase volume too far, then the pressure increase as the pistop drops will be so minimal that it wont force a charge into the barrel. But, those what know more than what I do open up their transfers, so I did too, but I haven't really gone very far with it. Mild tune.

Which leads nicely onto the case itself.
20180910_211222 by richardhenderson1968, on Flickr

I'm thinking about removing these "lugs". The only purpose I can see for them is that they keep crankcase pressure up, but they do hinder airflow through the case; or am I talking bollocks? I won't remove them unless I get good advice that it's of benefit. Pretty tricky to put back!

Forgot to mention that it was bored out to 62mm by ben at af on Saturday. He recommended 4 thou clearance so I just went along with that. A bit less running in. Also supplied an asso 2 ring piston.

I decided not to remove the 3 lugs for 2 reasons: firstly, the charge needs to pass over these from the inlet toward the transfers so they actually smooth the transit path (I reckon); and secondly all the images I could find on interwebs showed them left in place when tuned by the likes of MB etc. If it's good enough for them.............

So next step was to open up the casing transfers to match what I've done to the barrel.
20180913_180923 by richardhenderson1968, on Flickr

It's a bit of a waste of time as I still need to add a packer plate, but it'll be close enough and I can easily do a little more matching if I think it needs it.



Moving back to the barrel, I don't want to start working on the inlet and exhaust ports yet. That'll be better left until I've got the crank back and can measure what I;m doing rather than relying on pure calculation. So for now, I just calculated the exhaust port width I want to achieve. Using MB's tech pages he suggests 0.71 x bore diameter for an arc length around the bore, so 0.71 x 62 = 44mm.
20180913_184033 by richardhenderson1968, on Flickr

rather than go right to 44mm, these are marked short at 43mm so that when I add a nice soft edge on the ports, they'll not go beyond this figure. A trapezoidal shape may be hard to achieve for an amateur like myself so it may end up a bit more like the 'T' shape of an avanti TT kit.

So that’s a cast barrel? Using rudamentral kit did you say? Good show! I know how much I struggled on my Mugello and that Ali.
Shane

shane BBoys wrote:So that’s a cast barrel? Using rudamentral kit did you say? Good show! I know how much I struggled on my Mugello and that Ali.
Shane

Apparently the neighbours hate me.

I'm starting to have doubts about such a wide exhaust port. 44mm may be too large for the rings as I'm notvreally sure what they're made of. I'm going to try to speak to ben at af this afternoon, unless someone else can advise. MB suggests 0.71 x diameter for "good quality modern pistons with wire rings", but does that include an asso with 2x 1.5mm rings?

While I wait for my crank, I think I'll match the inlet to the barrel.

First I need to decide whether to use a 25phbl or 28phbh, I have both. The 28 will be better on a large block that I plan to do, so I'd rather use the 25 on this.

Will the 25 restrict this 175 much? It will be run with either an ancillotti or bgm clubman.