Checking piston to valve clearances on DOHC

Checking piston to valve clearances on DOHC

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fergus

Original Poster:

6,430 posts

231 months

Tuesday 11th October 2016
quotequote all
What's the best way to check p-v clearances on a DOHC engine?

I'm planning on running some fairly high lift cams for my engine, but even with fly cuts in the pistons, I'd still like to check clearances given the cam timing I'm planning to run. Is this a case of buttoning everything up, (using an old head gasket), connecting the timing chain, timing in the cams then gently rotating the engine?

Any other methods people can recommend?

CrutyRammers

9,911 posts

154 months

Tuesday 11th October 2016
quotequote all
That, with some plasticine on the piston top, which the valve imprints, so you can measure the static clearance.
IME you want to put something on top of the plasticine so it doesn't all stick to the valve. Oil sort of worked, greaseproof paper might be better?

Boosted LS1

19,115 posts

216 months

Tuesday 11th October 2016
quotequote all
Also, don't tighten the head down, leave it loose. Then if a valve does touch a piston as you gently rotate the engine the head will lift slightly and you may not damage the valve.

tapkaJohnD

1,435 posts

160 months

Wednesday 12th October 2016
quotequote all
CrutyRammers said:
That, with some plasticine on the piston top, which the valve imprints, so you can measure the static clearance.
IME you want to put something on top of the plasticine so it doesn't all stick to the valve. Oil sort of worked, greaseproof paper might be better?
WD40!

And unless you have kids/grandchildren, Plasticene may not be on your shelf. BluTac does as well.

JOhn

KiaDiseasel

83 posts

47 months

Wednesday 12th October 2016
quotequote all
The first step is very easy. Bring a piston to tdc. Put one inlet and one exhaust in the head on that cylinder either just held by the stem seal or put a bit of masking tape or a small rubber band round the stem to stop the valves falling out until the head is on. No cams or anything else. Put the head on the block with a used gasket and nip down lightly. Then measure the actual distance the valves can travel up and down to the piston with a dial gauge or just a vernier caliper. You should have a lift at TDC figure from your cam supplier and you're looking for a clearance at least 2mm more than this. This gives you a ballpark safe or not safe and whether the engine will actually even turn over when fully assembled without bending anything.

To measure the clearances exactly then the best way is to fit the two valves with very light springs. If you have nothing knocking about you can buy springs for a couple of quid. Something about 2 inches long and made from 1mm wire will do. Assemble with cams with the piston well down the bore and then gradually bring it towards TDC. Keep pushing the valves down and measuring the clearance. The exhaust will make its closest contact to the piston about 10 or 15 degrees before TDC and the inlet about the same amount after TDC. Conventional wisdom is a minimum of 1.5mm clearance on the inlet and 2mm on the exhaust but you can run a bit closer if you know what you're doing.

I really dislike messing about with plasticene or bluetac with the engine assembled with its proper valve springs. It's not very precise and it's a recipe for bending something.

Little Pete

698 posts

50 months

Wednesday 12th October 2016
quotequote all
KiaDiseasel said:
The first step is very easy. Bring a piston to tdc. Put one inlet and one exhaust in the head on that cylinder either just held by the stem seal or put a bit of masking tape or a small rubber band round the stem to stop the valves falling out until the head is on. No cams or anything else. Put the head on the block with a used gasket and nip down lightly. Then measure the actual distance the valves can travel up and down to the piston with a dial gauge or just a vernier caliper. You should have a lift at TDC figure from your cam supplier and you're looking for a clearance at least 2mm more than this. This gives you a ballpark safe or not safe and whether the engine will actually even turn over when fully assembled without bending anything.

To measure the clearances exactly then the best way is to fit the two valves with very light springs. If you have nothing knocking about you can buy springs for a couple of quid. Something about 2 inches long and made from 1mm wire will do. Assemble with cams with the piston well down the bore and then gradually bring it towards TDC. Keep pushing the valves down and measuring the clearance. The exhaust will make its closest contact to the piston about 10 or 15 degrees before TDC and the inlet about the same amount after TDC. Conventional wisdom is a minimum of 1.5mm clearance on the inlet and 2mm on the exhaust but you can run a bit closer if you know what you're doing.

I really dislike messing about with plasticene or bluetac with the engine assembled with its proper valve springs. It's not very precise and it's a recipe for bending something.
This method every time. With piston dwell at TDC, this is the only accurate way of measuring valve clearance as the exhaust valve is closing and the inlet valve opens. Leave the plasticine in the toy box!

KiaDiseasel

83 posts

47 months

Wednesday 12th October 2016
quotequote all
I might add for completeness that a check on lateral clearance to the valve cutout as well as the vertical clearance is very important, especially when fitting big valves. I've seen some horrendous mistakes in this area from sloppy engine builders with valves scraping away down the side of the cutout.

The way I've always done it is to take an old valve and cut the head off then sharpen the tip to a point on a lathe or grinder. Put the bare head head on with a piston at TDC and make a very light centre pop in the piston crown with your new valve tool down each valve guide. Take the head off and measure the piston from the centre pop to the side wall of the valve cutout. You want about 1.5mm radial clearance to account for valves expanding with temperature and moving about in the valve guide from side to side. With worn guides this can be more than you might think.

fergus

Original Poster:

6,430 posts

231 months

Wednesday 12th October 2016
quotequote all
KiaDiseasel said:
The first step is very easy. Bring a piston to tdc. Put one inlet and one exhaust in the head on that cylinder either just held by the stem seal or put a bit of masking tape or a small rubber band round the stem to stop the valves falling out until the head is on. No cams or anything else. Put the head on the block with a used gasket and nip down lightly. Then measure the actual distance the valves can travel up and down to the piston with a dial gauge or just a vernier caliper. You should have a lift at TDC figure from your cam supplier and you're looking for a clearance at least 2mm more than this. This gives you a ballpark safe or not safe and whether the engine will actually even turn over when fully assembled without bending anything.

To measure the clearances exactly then the best way is to fit the two valves with very light springs. If you have nothing knocking about you can buy springs for a couple of quid. Something about 2 inches long and made from 1mm wire will do. Assemble with cams with the piston well down the bore and then gradually bring it towards TDC. Keep pushing the valves down and measuring the clearance. The exhaust will make its closest contact to the piston about 10 or 15 degrees before TDC and the inlet about the same amount after TDC. Conventional wisdom is a minimum of 1.5mm clearance on the inlet and 2mm on the exhaust but you can run a bit closer if you know what you're doing.

I really dislike messing about with plasticene or bluetac with the engine assembled with its proper valve springs. It's not very precise and it's a recipe for bending something.
All makes sense, thanks.

The cam supplier has only given "traditional" timing info (i.e. In/Ex opening/closing in crank degrees) and nothing else (other than cold valve clearances). However, I can work out how much lift I get at the suggested timing info at TDC, then compare this to the max available clearance (at TDC), per your suggestion above. As also suggested, this will keep everything clean!

fergus

Original Poster:

6,430 posts

231 months

Wednesday 12th October 2016
quotequote all
KiaDiseasel said:
I might add for completeness that a check on lateral clearance to the valve cutout as well as the vertical clearance is very important, especially when fitting big valves. I've seen some horrendous mistakes in this area from sloppy engine builders with valves scraping away down the side of the cutout.

The way I've always done it is to take an old valve and cut the head off then sharpen the tip to a point on a lathe or grinder. Put the bare head head on with a piston at TDC and make a very light centre pop in the piston crown with your new valve tool down each valve guide. Take the head off and measure the piston from the centre pop to the side wall of the valve cutout. You want about 1.5mm radial clearance to account for valves expanding with temperature and moving about in the valve guide from side to side. With worn guides this can be more than you might think.
All good info, thanks.

99hjhm

404 posts

142 months

Wednesday 12th October 2016
quotequote all
Never had any luck with plasticine etc dial gauge works best for me. If it's closer than the figures Kia said above you might want to check it at Closest point of approach too. You really need cam plots for that. Depending on the engine you might want to check clearance between the valves too, that's usually on older 2v stuff.

spddm0n

1 posts

2 months

Thursday 25th June
quotequote all
Wow! Excellent explanation! Thanks! I was not able to find another resource for this anywhere on the internet. There are lots of videos about how to do this with traditional push rid V8 engines, but nothing for my inline 4-cyl overhead cam engines. I've tried several times with modeling clay, but I think my lateral clearances are pretty tight and this is causing the clay to tear, leaving me with poor results. By the way...WD40 is not a good lubricant for clay. It eats away at the clay pretty fast. Motor oil and assembly lube seem to work better.

Thanks!!!

KiaDiseasel said:
The first step is very easy. Bring a piston to tdc. Put one inlet and one exhaust in the head on that cylinder either just held by the stem seal or put a bit of masking tape or a small rubber band round the stem to stop the valves falling out until the head is on. No cams or anything else. Put the head on the block with a used gasket and nip down lightly. Then measure the actual distance the valves can travel up and down to the piston with a dial gauge or just a vernier caliper. You should have a lift at TDC figure from your cam supplier and you're looking for a clearance at least 2mm more than this. This gives you a ballpark safe or not safe and whether the engine will actually even turn over when fully assembled without bending anything.

To measure the clearances exactly then the best way is to fit the two valves with very light springs. If you have nothing knocking about you can buy springs for a couple of quid. Something about 2 inches long and made from 1mm wire will do. Assemble with cams with the piston well down the bore and then gradually bring it towards TDC. Keep pushing the valves down and measuring the clearance. The exhaust will make its closest contact to the piston about 10 or 15 degrees before TDC and the inlet about the same amount after TDC. Conventional wisdom is a minimum of 1.5mm clearance on the inlet and 2mm on the exhaust but you can run a bit closer if you know what you're doing.

I really dislike messing about with plasticene or bluetac with the engine assembled with its proper valve springs. It's not very precise and it's a recipe for bending something.

Zener

17,055 posts

177 months

Thursday 25th June
quotequote all
Checker springs everytime , you can use old rocker assy springs from a push-rod motor etc , this method don't llie or mislead