DRATV Piston Hitting ST90 cylinder head

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Shamrockwill

Member
Hello all. In my bored out ST90 top end rebuild I have run into a problem with my DRATV 53.5 mm piston. I have just installed a DRATV performence camshaft with help from this forum. However when I try to turn the flywheel it gets really hard to turn right before top dead center and when I force it...it jumps past TDC to the F mark, which I'm pretty sure means my piston is hitting the cylinder head. I'll have to take it all apart again and probably get a new head gasket and then figure out how to get my 53.5mm DRATV piston to not hit the cylinder head. DRATV warns about this problem and says to use an extra paper base gasket it it occurs. I don't think my gasket set had 2 paper base gaskets and I think I threw away the one it did have thinking a paper gasket was worthless...oops! Can I use 2 regular base gaskets? if not which side do the paper ones go on?

I did use a NOS OEM Honda base gasket which is thicker than the Vesrah gaskets that I have a set of, but I guess it's still not thick enough? I also just read something about the copper head gasket possibly not being big enough for a 53.5mm piston to go through. should I try to reem it out some with my dremel? all suggestions ar welcome and thanks ahead of time!

-Will
 
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Are you certain it's not making valve contact? Valves will deflect, providing that "spring action" you describe; head contact will stop the piston dead, it has zero "give". If they warn you to use two base gaskets, then that may be your best bet. If you know what you're doing, the best correction might be enlarging the valve relief in the piston...provided that there's enough material thickness. If a second base gasket is thick enough (~0.015") to eliminate valve-to-piston contact, there should be enough metal. Massaging the one contact spot will reduce compression less than a second base gasket, it'll also play nicer with the timing chain.
 
Shamrockwill said:
However when I try to turn the flywheel it gets really hard to turn right before top dead center and when I force it...it jumps past TDC to the F mark, which I'm pretty sure means my piston is hitting the cylinder head.

-Will
Click to expand...

It means the valves or at least one valve is hitting the piston. Which means you now have at least one bent valve now that you forced it and didn't stop right away. Best to take the head back off right now and replace the valve that is freshly bent and put a stock cam back in as this sounds like a hassle just to have a ''performance'' cam that most likely is more hassle than good. A performance cam really isn't good unless you have more than just a cam. In other words, more compression, bigger carb and a free flow exhaust.
 
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No matter what, top end has to come off. Get it off and work it out. It may just be hitting the metal ring of the head gasket, you might be lucky, no point worrying, just rip into it. Go from there. Assess the head amd valves while apart. Once you think you have it sorted, clay up the piston, put it back together and make sure you have decent clearance, add extra gaskets, reliefs, etc., i would choose more clearance over higher compression, i perspnally would rather loose a mph or 2 than have to rebuild it because it hand grenaded
 
Crap! Well now I'm extra worried. I hope I didn't bend the new valves. The cylinder has already been bored by DRATV for their 53.5mm piston kit, and I bought a CT110 carb to pair with it on their advice (I asked if it was a good choice to go with the big bore and performance camshaft and they said it was a great choice). Is the new camshaft what's causing the problem? This is my first top end rebuild (or serious engine work of any kind really) so I'm not really fit to make an assessment of much, I'm not familiar with reliefs and I also have no idea what claying up the piston is. I guess I'll take it all apart again and see what I can find. I really hope I'm not in over my head on this one.
 
Make sure the piston is in there the right way. The valve relief cuts are 2 different sizes for intak or exhaust valves. If it's upsidedown the intake valve might hit because the relief cut in the piston is too small.
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Whew! big sigh of relief. upon inspection it looks like motodevo's best case scenario is correct. the valves seem to be just fine but my copper head gasket is pushed in on over half of the inside and I just measured the inside diameter and it's definitely too small to fit the 53.5mm piston through it. Even with it pushed in I'm only getting just over 51mm on my digital caliper. Maybe I am fit to inspect things lol. Also I checked the piston and it's not installed backwords. Is there somewhere I can buy an ST90 copper head gasket with a bigger inside diameter?
 
Can you flatten it out gently with a hammer and then enlarge the inner with the dremel. Its just a copper ring on the st90 isnt it? By claying up the piston i mean put a thin layer(2 or 3mm) of modelling clay, play dough, blutac, or similar on the top of the piston, assemble topend of motor , spin her over, pull the top off again and see how much the of the clay gets squashed, you should not have any areas where the valves or head dome have squashed the clay to the point of seeing the top of the piston where you have put it. Id do it without the head gasket, that way you have a little extra leeway when you put it back together.
As far as a bigger gasket, from memory a stock ct110 one should be 1mm bigger than a 90 one, perhaps ask dr.atv, you can't be the first with this issue
 
Haha, I was just looking at that one and was going to post it (you beat me to it) and ask if the inside diameter is too big. I think it is for their 54mm piston but will it work for the 53.5mm too?
 
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Oops I didn't see motodevo's post before I replied. I'm going to assume that means yes it will work for the 53.5mm piston also. Thanks guys, that's twice now this forum's members have helped me out!
 
Yeah mate, I'd use that one. Although to be honest looking at dr.atvs website I think they supplied you the wrong gasket in your kit, they bore your cylinder to 53.5mm and supply a new piston to suit but gave you a smaller head gasket than your new bore. Order a few base gaskets while you are at it. Or you could just buy some gasket paper from an auto store and make your own, it's handy stuff to have around, a couple of $4 rolls in different thicknesses have served me well for about 10 years and will probably last another 10, that and it means that at 6pm on a saturday night when you realise you need a new gasket, you dont have to wait until monday morning(only to find they have to order them in) and rescues you from the tempation of using "liquid gasket" types of products to bodge it if you're the impatient type like me.
 
I'd contact dratv about the gasket...and probably order the right one, too, just to have a spare on hand.

Meanwhile, tape the top of the bore (to prevent metal-to-metal contact), then scribe the gasket with a small seal pick. That'll allow you to open-up the gasket to match the cylinder, getting the opening the right size and in the right location.
 
If I remember correct DRATV didn't send me a head gasket at all with their 53.5mm piston kit. Now that I am looking at their website I am correct. they have a disclaimer on the piston kit page saying you must dremel your head gasket bigger or buy the bigger one they sell. Problem is they do not have that same disclaimer on the page for the bore job and piston kit package deal that I purchased. Also says they include a 2nd paper base gasket which they did not. I guess I should have looked at the piston kit page. Oversight on my part and theirs I suppose. I'll just order the one they sell as I also need a new battery cover anyway. Thanks again guys!
 
Very logical...and reasonable.

Just a thought, before pulling the trigger on that next parts order, try spinning the crank over, without a headgasket in place, and the head sitting atop the cylinder sans nuts. That'll provide one more clearance check and it's failsafe.
 
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