Piston hitting cylinder head

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Mat
Cucciolo - the Lil Pup
Posts: 15
Joined: Wed Nov 27, 2013 6:36 am

Piston hitting cylinder head

Post by Mat » Tue Jan 21, 2014 3:49 pm

After finally getting the head off of my 250 Monza I notice it looks like the piston has been kissing the cylinder head chamber. Head surrounding surface as well as the top of the piston has a pitted look, possible from carbon getting pressed between the two, I can also see a bit of an imprint of the intake valve seat on the piston. Leak down test on engine before pulling head showed intake valve leaking so maybe piston hit valve and bent it but it is not noticably so. I will have to work on getting valves out. There is a thin cylinder base gasket in place, no head gasket. Can head gaskets be used? The only thing I can think of right off for this tight clearance is that the head seating surface was milled too much. Any input appreciated, thanks.

ducadini
750 Sport [BEVEL]
Posts: 220
Joined: Sat Jul 31, 2004 8:54 am
Location: Antwerpen

Re: Piston hitting cylinder head

Post by ducadini » Wed Jan 22, 2014 1:22 am

Just lap in the cilinder/head surface and fit a thicker base-gasket.

ciao,
ducadini
Nego i ricordi peggiori
Richiamo i migliori pensieri

Mat
Cucciolo - the Lil Pup
Posts: 15
Joined: Wed Nov 27, 2013 6:36 am

Re: Piston hitting cylinder head

Post by Mat » Wed Jan 22, 2014 3:08 pm

Thanks, a thicker base gasket sounds like an easy fix. I thought about lapping in cyl/head but the nipple sticking up from the cylinder for oil would be in the way, does this come out?

ducadini
750 Sport [BEVEL]
Posts: 220
Joined: Sat Jul 31, 2004 8:54 am
Location: Antwerpen

Re: Piston hitting cylinder head

Post by ducadini » Wed Jan 22, 2014 11:28 pm

Yes that dowel can be pulled out.
You won't be able to turn the head a full 360 (not even close) due to the exhaustport sticking out.
Check with engineers-blue if the head isn't touching the cilinder in other places then between cilinder and head.
Try to avoid that grinding paste is caught between head and cilinder on the sides of the cilinder, try to keep that a easy interference-fit.
Carefully try to match the innerdome of the combustionchamber to the sealing surface, there shouldn't be a ridge.
I've had some base-gaskets made of aluminium-plate to compensate for too-high pistons.
Before beginning , fit everything (you could leave the rings of the piston) and put some small strips of moldable plasticine (putty ?) on the piston dome.
Tighten head, turn engine over a few times (slowly) and open.
You should be able to tell where the head-piston clearance is critical and how much.
Under 0.8 mm and You're in for some headaches :-).

have fun
ducadini
Nego i ricordi peggiori
Richiamo i migliori pensieri

Mat
Cucciolo - the Lil Pup
Posts: 15
Joined: Wed Nov 27, 2013 6:36 am

Re: Piston hitting cylinder head

Post by Mat » Sat Jan 25, 2014 2:08 pm

Thanks Ducadini, sounds like good advice. I got the oil dowel out. Now I am working on getting the valves out to see what is up there. If I am lucky they can be just lapped in but I will see. Hope to use the cylinder/piston again. Piston looks OK with just some bit of scuffing which I hope will clean up, rings looked to not be on quite right as upper oil ring had markings on the top while lower oil ring had the markings on the bottom. I read ALTO should face up, I am not sure markings say ALTO but I would think they should face up. I can't see any markings on compression rings but I do note they have different cross sections. Rings don't look to have much use on them, I was wondering if I could just reuse them if I put them on properly. Cylinder not great but also not bad and within spec's I believe.

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