GT big block experiance : New GT crank info added

[joespeed]

any heat issues with this kit,i did ask 'why cast' and not alloy,surely the cast is going to retain the heat even though you are using the 'porqupine head' and at high rpm its going to run hot.

[Rich_T]

Casting in iron presents so problems that are not immediately obvious. The principle problem is that it is much hotter than alloy and consequently the surface of the iron can sometimes have a residue of sand and shot blast media from the foundry cleaning process.

Additionally, you do get some flashing between core joints, this is particularly evident where the boysen ports join the reed chamber. This flash has to be removed with a small burr. Normally, I can design the core layout so that flash is rempved with the machine operations but in the case of the boysen ports this was not possible. The rest of the port walls are just flicked over with a stone in a pencil grinder to be sure that the port walls are clean, there is not heavy grinding and the surfaces are quite smooth (the flash on the camera does not show this well). While I'm at it I also check the reed valve fit and as this is the first batch the fit is a bit too tight, the CNC program will be adjusted for the next run. So the answer for Spanish fly is, yes, this is how they come.

All porcupine heads have two spark plug positions and a blanking plug with an M8 thread for fitting a thermocouple, these are a production item and have been for some time now.

[Rich_T]

Why cast iron? Well it isn't that long a story but it does require an independent mind to separate the wheat from the chaff.

Lambretta air cooled barrels are cooled asymmetrically, This means that the cooling of the barrel is not symmetrical left to right around the throw of the con rod. The result is that as the cylinder gets hot is it distorts on a banana shape slightly to the left (as this is where the cooling charge comes from). To make matters worse this distortion is not in the plane of the con rod. Consequently as the barrel distorts the bore and piston tighten up.

The coefficient of aluminium is much higher than iron, therefore iron does not distort anywhere near as much so an iron barrel is straighter. The expansion also puts a strain on the cylinder studs which necessitates re-tightening. Iron is also much more reliable and robust for secondary operations (machining and cylinder walls) but contra to common belief tooling and casting in iron is more expensive than alloy and the technique, knowledge and skill far harder to come by.

There is much more on this and many other production engines that you can draw examples and comparisons to. The long and the short of it is, expansion. I'm pretty certain Innocenti new all this and that is why in a factory where they could easily have made a lined alloy barrel they stayed with iron. The confirmation comes in the form that virtually every 40 - 50 year old Lambretta that gets imported over from Italy always has an iron barrel and typically in 5 minutes with a new plug and a can or easy start, they run. As a statistical proof of concept it doesn't get much better than that.

[speedy]

A genuine question here,as I'm thinking seriously about this kit now for my other gp later in the year.Of the two kits I was looking at for touring,this seems to be in the top one As I canna remember seeing a gt kit[186 or this one] with the porky head,does it need a special head cowl,or even not use one? cheers,Speedy.

[lammy57]

Hi Eden not put it on yet! went down to do it on sun but instead of just new bearings and seals I now need a new crank stator and flywheel oh the joys! all ordered now so hopefully this weekend coming

[Rich_T]

A genuine question here,as I'm thinking seriously about this kit now for my other gp later in the year.Of the two kits I was looking at for touring,this seems to be in the top one As I canna remember seeing a gt kit[186 or this one] with the porky head,does it need a special head cowl,or even not use one? cheers,Speedy.

Porcupine heads are a touch different in a couple of ways. firstly the side spark plug hole is directly opposite the exhaust port (10 degrees off from the vertical plane). This is not as much as the original spark plug which is more like 12-15 degrees. This means a plug spanner can be quite a tight fit on some head cowlings so it might be worth opening up the hole a tad. Secondly,

The head thickness is thinner than an original head combine this with a GT kit and you will need to look for a couple of things, the first is that your cylinder studs do not run out of thread at the top (series 1 studs are fine, longer studs might need spacers). The second thing is that you should use 4 standard cylinder head nuts, then fit the long cowling nut on top of the 4th cylinder head nut. The extra thickness of the nut off sets the length of the cowling nut so it supports the head cowling properly. At some point I'll make some special cylinder head nuts to cover these issues at source.

Yes, GT kits and porcupine heads need a cowling

[byron]

...This means a plug spanner can be quite a tight fit on some head cowlings so it might be worth opening up the hole a tad.

I had to do this on an innocenti series III cowling [with inlet manifold air scoop], I elongated the hole and curved the edge over as original. I did this before paint, it is not really noticeable now. [Dry-build is your friend]

[eden]

Ive not yet had to open the spark plug hole up on the cowl on the three ive built, its a fiddle to get the cowl to sit right but it is possible.
The cowl obviously needs cutting around the reed just like any other reed kit.

[speedy]

A genuine question here,as I'm thinking seriously about this kit now for my other gp later in the year.Of the two kits I was looking at for touring,this seems to be in the top one As I canna remember seeing a gt kit[186 or this one] with the porky head,does it need a special head cowl,or even not use one? cheers,Speedy.

Porcupine heads are a touch different in a couple of ways. firstly the side spark plug hole is directly opposite the exhaust port (10 degrees off from the vertical plane). This is not as much as the original spark plug which is more like 12-15 degrees. This means a plug spanner can be quite a tight fit on some head cowlings so it might be worth opening up the hole a tad. Secondly,

The head thickness is thinner than an original head combine this with a GT kit and you will need to look for a couple of things, the first is that your cylinder studs do not run out of thread at the top (series 1 studs are fine, longer studs might need spacers). The second thing is that you should use 4 standard cylinder head nuts, then fit the long cowling nut on top of the 4th cylinder head nut. The extra thickness of the nut off sets the length of the cowling nut so it supports the head cowling properly. At some point I'll make some special cylinder head nuts to cover these issues at source.

Yes, GT kits and porcupine heads need a cowling

Thankyou for the indepth and helpful reply,that has answered my queries. Will there be a possibility of a purpose made head cowling in the future? That would be handy for those of us whom are a tad heavy handed with a jigsaw and grinder

[Rich_T]

In short no, even a composite one would be way down the priorities. The most economical way is to 5 axis laser cut an Indian cowling but the problem will always be cost compared to the 5 minutes with a cutting disc and file.

Faced with a £35 laser cut Indian cowling or 5 mins with a grinder, 99.9% of people will reach for the grinder.