Question about decomp hole plug upz

so i have this big ol cylinder guy, and i want to plug up the decomp hole which goes from the head to the exhaust port to use my custom head.

Honestly, the only effective lifetime fix for this that i can think of is to tap the hole, then insert a threaded set screw with some sealant or epoxy on the threads. Simple right?

However, i am worried that the act of tapping this hole and plugging it, will put outward pressure towards the cylinder bore. Much light over tightening the intake bolts on a polini puch cylinder. Notice how little room i have between the hole and the actual bore.

Another idea i had was to drop this off at my machinist and have him tig up the hole itself in the exhaust port, and smooth it out.

BESIDES FILLING THE HOLE WITH JB WELD OR WHATEVER OTHER GOO YOU DUDES LOVE, anyone have a real idea on how to plug this up? Think tapping it will be ok?

Re: Question about decomp hole plug upz

much like*

Re: Question about decomp hole plug upz

Honestly dude, I've been running JB weld on a puch polini for over 3 years and it's good. I know Chris Rice has had JB blocking off an entire port on his Vespa Sprint for at least 4 years...don't discount the JB man. Yes, many many people use it incorrectly, but if you use it properly you can have amazing results.

Don't tap it, if you do that you will probably do exactly what you are trying to avoid, fuck with the bore...tig it or jb that shit right up.

Re: Question about decomp hole plug upz

I've tapped and plugged the decomp hole on my puch kit, but you're right, yours is WAY closer. While in my head, I don't think tapping it out and plugging it would put weird stresses on it like the polini intake problem because the hole you're tapping would be parallel to the cylinder bore as opposed to perpendicular to it. That said, things that 'work in my head' are not something I'd expect you to try on a cylinder that costs over $100, and probably more.

Just another idea to try, would be to find a small piece of rolled metal bar stock that fits in the hole, and either epoxy it in, or like you said, put a lil' dab of weld at each end to keep it in there, and then grind, mill or sand it flat.

Wicked portmap, BTW. What kit?

Re: Question about decomp hole plug upz

I tapped the deconpression hole on one of my puch 65 metras and it had been overbored twice. The hole actually looked closer to the bore. Make sure to get that tap strait and keep it lubed. I threaded in a set screw with high temp rtv in the threads, then I covered the top of the screw with a little more RTV sealant just to make sure it sealed off tight.

I havn't had any problems yet. Actually, I solved the air leak I had because the bore was so close to the hole.

good luck,

Eric

Re: Question about decomp hole plug upz

why are you cloggin the hole ?

Re: Question about decomp hole plug upz

i know you said its for your custom head.

but what reason would any one have to do that . what does it do

Re: Question about decomp hole plug upz

I would try to find a tapered pipe plug tap that's close or a set screw and then jb weld over the top of the plug. That way you can always go back to using the decomp if you want. Check for distortion by inserting your piston and running feeler gauges around it. A hone would also show a high spot if there is one.

Re: Question about decomp hole plug upz

I think tap and a set screw would work well, and use some red loctite on that thing. The only kind of torque problems warping the bore I could see happening are if you use a really long set screw and bottom it out against the back side of the cylinder. With a short screw I doubt it would have that problem.

Re: Question about decomp hole plug upz

I don't see why it would distort the cylinder. The heat from TIG welding the hole will produce much more distortion than tapping the hole. If you are using a custom head then why don't you just make sure the head covers the hole. Make a head gasket that doesn't allow air through the hole. The only reason you would plug it is it makes an air leak in the exhaust and makes noise, so if the head covers the hole then why worry.

Re: Question about decomp hole plug upz

I would just tap it.

The heat from the weld will be pretty intense too, but I've welded studs into my scooter's polini kit.

Re: Question about decomp hole plug upz

I would just tap it and run a set screw down in, but I'm not really sure whats best. I think the intense heat JB weld would see so close to the inner bore would probably break it down. I also think welding is more likely to distort the bore then taping, that I'm fairly sure about.

-naz

Re: Question about decomp hole plug upz

i had my motobecane cylinder tig'd up like crazy, and it was fine. Good point though, this is obviously too close to the bore so welding is out of the question.

There have been some good ideas on this thread thanks for all the suggestions. I will look into my tap collection to see if i have the perfect one.

Re: Question about decomp hole plug upz

Drew and Jesse, i am trying to completely eliminate this hole from any possible air leaks ever.

Re: Question about decomp hole plug upz

Fair enough, I put locktite on the setscrew when I put them in, and I don't tap the hole all the way through so there is no way in hell the screw can drop down into the exhaust port. Seems like a good idea. The only thing I can think of being an issue is being that close to the bore is the rate of metal expansion due to heat will be different from the cylinder and the setscrew, and may cause the cylinder to be tight there. But we're talking super small amounts that'd I'd write off and not worry about.

Re: Question about decomp hole plug upz

i also thought about expansion rates.. but i think that would be an issue no matter what material i used.

Re: Question about decomp hole plug upz

Yeah it's such a small amount of metal it shouldn't expand that much. Good luck!

-naz

Re: Question about decomp hole plug upz

I was thinking aluminum set screws, but they don't seem to have any on mcmmaster, so why not some aluminum all thread of the right size cut off and slotted. or maybe an aluminum dowel pin, slid in and epoxied

Re: Question about decomp hole plug upz

great idea chuck! i can always thread some alu rod through a die and make my own!

Re: Question about decomp hole plug upz

crackstar, I think you've hit upon the correct solution. Aluminum rod epoxied in place would be almost like not having the hole at all. Absolutely no risk of distortion. And if it does come lose, it would only slightly block the exhaust port with little ill effect.

use the crackstar method. :-)

Re: Question about decomp hole plug upz

could you put a piece of metal round stock thats like half a millimeter to big or bigger then lathed down.

then freeze the plug and put the cylinder in the stove. and it should block it with no holes.

Re: Question about decomp hole plug upz

you talk of this machinist... I need to know of him.. I need my puch case welded up sooon

« Go to Topics — end of thread

Want to post in this forum? We'd love to have you join the discussion, but first:

Login or Create Account