Modern Vespa

I can cut a stock cowl to fit over the MRP billet head, but I already messed up one attempt. Does anyone know if an 'off the shelf' cowl is available to accommodate this head shape and the centre plug?

If you find one let me know. Great head but really awkward.

I really hate it when they sell stuff that you need to figure out the rest. Unfortunately, no PnP cowl, you just have to cut it right...

Anyone?

Re the topic: I'm not there yet, but I see what needs to be done. I've had to play dr. frankenstein on another cowl for a Pinasco 225 with central spark plug, and I used a worn-out cowl for grafts, an exacto knife to cut out the new spark plug hole, and a soldering iron to melt a round graft from the old cowl onto the old hole. Hopefully I'll a neater job this time around

Regarding the MRP cylinder head itself: I just noticed that the threaded part of the spark plug sticks out in the combustion chamber by about 2.8mm, instead of sitting flush.
Doesn't seem right, and certainly isn't consistent with any of my other cylinder heads.
Are your setups the same?
.

Which plug? If it's an ES (NGK), then just swap for a HS which is the same but shorter.

It's a Denso IW27, which is a long-threaded plug that fits P200 heads and T5 heads, and is supposed to fit this particular head as well. Threads in typical cylinder heads machined for long threads are around 17.32mm long; this one is 14.2mm - quite a difference. A short threaded plug would ignite inside the plug well, which can't be good.

I sent off a message to mrp and asked; I'll see what they say.

No direct reply from MRP, but SCK told me they'd reached out to MRP and been told that the plug's protrusion in the combustion chamber was normal.

Head torqued on, the squish is 1.15 at the step at the edge of the piston, and 1.35 elsewhere. The distance between spark plug and piston dome is 5mm at TDC, which seems unusually close, but isn't unheard of in the automotive world. We'll see...

There should never be any threads showing. Plug or head. They are hot spots - and will cause pinging.
Suggest you get longer plug to take up all the threads.

Cowl: https://www.sip-scootershop.com/en/product/cylinder-cowling-quattrini-vmc-bfa-244-tomas-compositi_23387600?q=Vmc%20cowl

Expensive but it exists.
Maybe others?
VMC kit comes with a rubber plug to fill hole of existing cowl, then you cut a new one. Perhaps they sell just that plug. If not, Amazon sells rubber plugs that could fit the cowl hole - when you cut a new central one.

The hole for the central plug and the bung for the old hole are easy to do. The problem is with the lower cut to accommodate the odd shape/bulge of the MRP heads. I have messed up one of my stock cowls trying to get it right. The Scooter Center picture with the red line drawn by a child is unclear. If only they provided a template, or better still, a modified cowl that I could just buy. I don't care for carbon, kevlar or titanium for that matter. A black plastic one with the correct cuts would suffice

I have the race version of this head on my 221. 0.8mm squish, hardly any cowl left and no bungs, just open. Full side panel. Still holds steady under 280f at motorway crusing speed. Wouldn't worry about it too much. These heads are good.

I think you mean a shorter one CM?

Other reason not to have too long a thread is that the threads will build up with carbon and ruin the threads in the head as you go to remove it... which is obvious I guess.

The ES in the NGK is too short at 12.7mm vs the nominal 19mm of the Denso, perhaps copper indexing washers might be the only answer. Or just a plain 2mm(?) copper washer, though they tend to have a wider OD than the indexing washers. Can't find a plug that has a 15mm thread reach.

I meant shorter - but it was because I misread the photo and thought I was seeing the plug inset rather than proud.

Second look - Ginch is on the money.

Indexing washers seem an excellent idea here, inexpensive and easy to execute. Amazon sells them by the dozen in various thicknesses.

Good to know, thank you. I'm assuming the 0.8 squish figure is due to yours being the race version? The figures I get (1.15 at the edge and 1.35 elsewhere) are with no gaskets whatsoever, just sealing compound, but it's the touring version.

I'll look into the 2 mil washer idea; I'll feel less paranoid with the plug at its regular position; i.e. flush with the wall of the chamber. Thanks for the suggestion.

Regarding the cowl, I've started working on mine with a heat gun, but it's slow. I'll post a pic if I get it to fit right without cutting it up.

I placed an order with sck for some missing doodads, and I ordered a copper washer for 14mm spark plugs, with a thickness of about 1.8mm. Close enough.

And I finally got around to taking care of the cowl. The hardest part is getting the hole used to screw the cowl into the rotor cover and the crankcase to stretch out far enough to face the fixture in the crankcase.

First I closed the original spark plug hole using a piece of plastic from a donor cowl and a soldering iron:

Then I tried various manners of heating the plastic to stretch it across the cylinder head fins and towards the rotor cover with a heat gun. It's pretty tense, but I'm finally able to reach the screw I inserted from inside to make it easier. The fit isn't perfect, as you can see the top holes on the side don't match:

Then when I was satisfied with the fit (the cowl screws onto the spacer nut on the cylinder head, and into the crankcase at the rotor cover), I screwed the plug in, put a dab of grease on the tip to mark the inside of the cowl, and proceeded to drill with a stepped drill bit.

Nice job!