Stock Puch Head On Hobbit

I know there are way better heads to match up to hobbit for better cooling but I’m really in budget mode right now and have two 39mm puch heads I can rip into. The one on the left has a squish band but smaller fins. Looks like it might be easier to slot the holes too. The one on the right has larger fins but no band and the slotting might get tricky with those raised tabs? Out of the two, which would you choose?

Full disclosure, I’m running a DR, proma, boison stock reeds, and vm20.

Also, shoutout to JBOT for the good puch head matching thread from a while back. Had a lot of good info in that thread.

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Re: Stock Puch Head On Hobbit

I have used both following Jbot’s slotting method.

The magnum head works fine but I decked it so it was completely flat and had to trim the fins on the bottom for the Proma.

The regular 50cc ones well and are my favorite to use. I just slot them and cut the squish with a dremil.

Re: Stock Puch Head On Hobbit

Eric, are you cutting the squish to fit the larger piston, or what?

Re: Stock Puch Head On Hobbit

Beware the bolt depth.

You MUST add a significant amount of spacers under the heads of the bolts in order to not bottom out the threads are break em off or strip the holes.

You have been warned

Re: Stock Puch Head On Hobbit

> Pat Jack Smog Wrote:

> -------------------------------------------------------

> Eric, are you cutting the squish to fit the larger piston, or what?

Yes I used the hobbit gasket for an outline. Then cut a mild squish I forget the measurements just so the gases would burn more evenly.

I totally forgot about what Jbot said about the spacers. That is a must with the bolts. I changed mine to Puch studs.

Re: Stock Puch Head On Hobbit

Ok, so I got the holes opened up. Now to do the squish I suppose? I marked it out with the gasket, but when I assemble it I’m just going to use Hondabond on it. Hoping I don’t get any piston slap

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Re: Stock Puch Head On Hobbit

If you have a drill press you can chuck up the spark plug and spin the head, and attach a tennis ball with very rough sandpaper under it. That would make a custom squish in no time at all.

But be careful cuz spinning shit can make you bleed

Re: Stock Puch Head On Hobbit

Unfortunately I only have a flex shaft and handheld power tools. But I’m taking my time to get this nice and even.

Re: Stock Puch Head On Hobbit

Alright, time to slap this puppy on and see how it goes.

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Re: Stock Puch Head On Hobbit

Check squish and either mill the head or add a thicker gasket or spacer to change height

Re: Stock Puch Head On Hobbit

Well I checked the squish and all seemed well! I put her on and...success!! Great compression and the bike runs great.

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Re: Stock Puch Head On Hobbit

Emil Kniemel /

What size tires?

Re: Stock Puch Head On Hobbit

They’re 2.75 golden boys. They just baaaaarely fit the subframe, but they do fit. Definitely can’t go any bigger than that though.

Re: Stock Puch Head On Hobbit

Nice work dude!

Re: Stock Puch Head On Hobbit

How’s it performing?

Re: Stock Puch Head On Hobbit

It’s running great, I took it out for a whirl around town and it was performing great. When I got it back to the shop I checked the torque in my cylinder bolts and I noticed they were all looser. So I started to tighten them and realized the spacers I made were getting warped from the torque. I made them similar to the ones JBOT made in his original post, but I think I’m just going to buy some now so they won’t get deformed.

Other than that, it ran fantastically. I’m sure the one that treats offers is better since it’s machined, but this is a good alternative if you’re on a budget.

The other thing you have to figure out is making sure it’s perfectly centered when you put it on your cylinder. Since it’s slotted it’ll move around on you a bit when you’re bolting it on. But maybe that’s just because mine isn’t perfectly slotted?

JBOT’s that took the idea from:

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Mine after torquing them down too much:

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Re: Stock Puch Head On Hobbit

I’ve got some Puch studs I can pull from an engine so might just put those in there instead of the bolts.

Re: Stock Puch Head On Hobbit

I also used more specially cut triangle washers to make sure the spacers don't dig into the aluminum over time. I had some do that and got an air leak and detonated a hole in my piston.

Btw, you're tightening the bolts way too tight if those things did that. Use a baby 1/4 wrench for studs, like no longer then 5 inches or so (edited)

Re: Stock Puch Head On Hobbit

Good advice, thanks JBOT!

Re: Stock Puch Head On Hobbit

JBOT, what do you mean by cutting triangle washers and how does that prevent cutting into the aluminum?

Re: Stock Puch Head On Hobbit

Thin spacers up against the head can dig in to the aluminum like a saw, every vibration drives them deeper into the soft aluminum.

Ill fitting small washers can cause this too, especially if the slot is slightly sloppy and enlarged like every slot is. When you clamp over a slot, you're only clamping on the sides, there is no equidistant clamping area. So I add as big of a washer and thick as possible too as I can to hit as much head area over a slot as possible without it warping under torque.

So I try to spread out the torqued pressure over as large an area on the head as I can. But when you slot a puch head the fins get in the way of having big washers. So I just cut washers to fit the space available, and trim the bottom of the fins as well for more space. (edited)

Re: Stock Puch Head On Hobbit

I see what you’re saying now. I went ahead and just picked up some steel spacers. I grabbed some oversized washers too and they definitely cover more space. I’ll have to cut them, like you said, to allow the bolt to be in the proper area of the slot.

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Another thing I did was I took a sanding disc and my flex shaft and sanded the top of the slots down to make it as flat as I can so the washer will sit flush. They were fucked up and gouged from opening them up. Now they’re silky smooth.

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(edited)

Re: Stock Puch Head On Hobbit

Probably more pics than necessary but I’m bored at home with the quarantine going on.

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Re: Stock Puch Head On Hobbit

That's exactly what you should have done. Hopefully it will have you trouble down the line.

Other stuff! What is this head going onto? You might have to thread and plug the decomp channel in the cyl unless this head's Matt by surface covers the hole. I'd also flatten the heads mating surface on glass or flat something with sandpaper to be sure of a great seal

Re: Stock Puch Head On Hobbit

Shit, that’s a good point about the cylinder decomp. I didn’t even think about that. It’s a DR and It looks like the head’s mating surface doesn’t cover it completely. Is there anyway I can plug it without threading it? It sounds shitty but I have hi temp quiksteel that might work. I guess I just don’t want anything getting sucked into the cylinder.

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Also, I plained the mating surface a bit make it nice and flat. (edited)

Re: Stock Puch Head On Hobbit

You have to tap it if it doesn't close up. Don't rely on epoxy. You could also find some rod thats oversized and smash it in then grind flush. I prefer tapping

Re: Stock Puch Head On Hobbit

I’m worried smashing that might warp the cylinder. The decomp is like 2 mm away from the cylinder.

Re: Stock Puch Head On Hobbit

Maybe have it welded shut ?

Re: Stock Puch Head On Hobbit

Shit I did not even think about this. Just because I always plug the decomp with an alan type set screw. I forget the size but I tap it and run that screw down in the decomp hole the put a little JBweld over the top.

Re: Stock Puch Head On Hobbit

. (edited)

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