Measuring piston to valve clearance opinions

[4AGEMAD]

So heres my jam, I am currently building a 4age 20v with 284/278 duration - 9.3mm/8.5mm lift cams, high comp pistons, 0.8mm steel headgasket and machined block & head.

My reasoning for wanting to measure my p/v clearance is that i'd like to retain VVT to help with the now would-be lack of power down low. 

 

Now Kelford state that VVT can be retained but with the extras that i'm adding i'd like to be 100% sure that nothings going to get all fancy up in there. 

I have been doing some research on this and have found a few different ways in doing this, each have their own pros/cons and each party bagging each others opinions, i'm not wanting to start that here, just wanting your opinions/reviews on a collective, ideally to have been done on a flat tappet styled engine similar to mine. 

 

The most common way I found was to use playdough or the likes & place a thin layer of it on top of the piston. Using an old head gasket (which i dont have) sit the cylinder head on top of the block and turn the crank over to TDC #1 followed by turning the cams 360 degrees. 

Remove the head, cut a small indented section of the playdough out and measure its width with a set of verniers.

 

Would this be a reliable way of measuring p/v clearance taking into account heat expansion etc at high rpm's.

All opinions and views welcome.

[CorollaGT]

If it's of any help James I have my old used TRD headgasket in the shed you can use

[SOHC]

We used plasticine and verniers. at my old work like you say

[Unclejake]

Plasticine is the best way.

 

Zero clearance with the rotating method you mentioned would only be interesting in a cam drive failure scenario though. It's better to know what the clearances are like with the cams and pistons moving in sync (I don't know your engine at all)

 

I'd also want the head torqued down on the gasket, and would allow some extra clearance for heat stretch and/or big end wear. AFAIK you can torque down a new gasket, and then remove and re-use it if it hasn't gone through many heat cycles

 

I once had a race engine that blew head gaskets at 82 degrees C, but by the time we got towed into the pits it was running sweet again. It took three gasket changes to work out that the piston crowns were kissing the head when the conrods got 'long' enough due to heat. 

[4AGEMAD]

That will be great thanks mark.

Cheers for the input guys,that was the stuff I couldn't remember.

I hear you there unclejake, I now remember the majority of examples I saw were with push rod engines rather than cambelt so makes sense.

Yeah ive herd mixed opinions about it aye, but if I could borrow corollagt's one thatll even better.

[Roman]

The valve cutouts in the piston arent necessarily there to make sure that you can rotate the valves 360 degrees without hitting the pistons.

As at TDC your valves will normally not be fully open so the cam timing keeps the two things out of phase even if they hit together if there isnt a cambelt.
 

If you've got higher lift/duration than standard, it will likely be an interference engine now anyway. (assuming it wasnt already)

 

Many motors are like this from factory... These are the ones where cambelt failure means an exploded engine, rather than a car that stops running haha.
 

Probably the best way is to setup the cambelt and timing as you're intending, then lock the crank angle in place while at TDC on #1 then turn the cam with a cresent through its range of motion for VVT. (assuming this doesnt do anything bad to the pulley)

 

Then check the clearance based on that.

[phatt20]

iv always used the plasticine method , dummy assemble engine with plasticine, coat valves in oil to stop the dough sticking, wind it over a couple of times (carefully) , strip it, cut the plasticine thru the middle of the valve imprint, and measure the thinnest point with verniers. built proly a hundred engines this way and they have allbeen fine. some have been pretty tight tolerance high comp engines (5k with flat top forged pistons/twin squish head @ 13.2;1 sorta thing)

[KKtrips]

Does the VVT work on the exhaust valve? I thought it would only on the inlet but I am not at all conversant with VVT technology.

[CorollaGT]

Does the VVT work on the exhaust valve? I thought it would only on the inlet but I am not at all conversant with VVT technology.

 

Only the inlet on these

[kpr]

pretty much need to assemble, and time it up with cambelt on. then you will need to sort out a way to hold the vvt pulley advanced.  as the advance position, is where the valves are going to get closest to the pistons.   then rotate engine.

 

check your piston to head clearances while you're  there, since using thin head gasket and block as been skimmed.

[Roman]

iv always used the plasticine method , dummy assemble engine with plasticine, coat valves in oil to stop the dough sticking, wind it over a couple of times (carefully) , strip it, cut the plasticine thru the middle of the valve imprint, and measure the thinnest point with verniers. built proly a hundred engines this way and they have allbeen fine. some have been pretty tight tolerance high comp engines (5k with flat top forged pistons/twin squish head @ 13.2;1 sorta thing)

 

The problem that he has in this case is that VVT makes it more complex.

When he winds it over by hand the intake valves will be in the retarded position.

 

But when the VVT advances the cam, it brings the valve timing forward by 20(?) degrees or so, and it might be during this advance that there are piston to valve contact.

 

A good example - a while back someone turbocharged a beams motor, using low compression pistons from an engine that had the valve cutouts on a different angle to the beams head. Checked the clearances with plasticine, all good.

 

However got the motor running and started tuning it... Advanced the cam timing and boom. pistons ate all of the intake valves.

[Rookie]

Could you not just put the cam belt on the with the intake advanced 20° ( or how ever much max advance is) and then you don't have to worry about making the vvt work?

[CorollaGT]

Fail post

[Roman]

I think he's meaning just for the sake of bench testing the valve clearances with cam advance, not running the car that way.

[Rookie]

Exactly what Roman Said.

[4AGEMAD]

Vvt advances the intake camshaft 30° taken at the crank or 15° at the cam pully.

Really appreciate all your input thank you guys.

So by gathering all of the information, the best way it seems to go about this is:

Bottom end assembled sitting at btdc #1, plasticine on piston dome, with old headgasket torque down head, assemble cams and set the intake cam as close to if not more than 15° advance and fit on the cambelt.

(Obviously will be all timed up correctly) wind the engine over, remove head and see results.

Sound about right?

Another thing, what would be an ideal clearance measurement you would see on a high comp engine?

[Rookie]

Wiseco say:

 

.080” intake side, and .100” exhaust side

[downtrail]

Yup that ^ unless you can pull apart vct pulley and operate it manually at tdc

[4AGEMAD]

Cheers rookie.

@downtrail: unfortunately the vvt pully is all pressed together and spring loaded.

But never mind, placing it at full advance then fitting the cambelt should do the trick

[Celica RA45]

the biggest problem with 20 v motors is the center lobe of the cam ,this will dictate how much you have

if the cam set ups are similar to the beams just set it up at tdc using spanner on cam ,advance ,holding crank with spanner ,using dial indicator on the bucket .check how far it moves ,then take belt off ,the inlet cam and keep advancing the cam till it hits the piston .check to see how much you have

as in vvt movement then after this see if you have 80 thou after that ,thats a minium that i use for piston to valve clearence

also note the last 15 degress coming up to tdc is also very critical .piston and valve gets really close here