Valves contacting Piston Crowns

[andyhodg]

Further to an earlier post regarding wear on the cam followers I have just started removing the cylinder head from my +2. My new concern is that on inspecting the piston crowns there has been contact between the piston crown and the exhuast valve on all pistons. The contact is just on the outside edge of the pocket machined into the crown for the exhast valve. I had the most recent work done on the cylinder head about 10 years ago (30,000 miles). I had new seats and valves fitted by QED. The head was re-worked by (ported and flowed) Vegantune many years before it was in my possesion. QED fitted 1.4" exhaust valve to the head. In my ignorance I didn't consider the pistons and QED didn't have sight of the pistons. I can't rember what size the original valves were.

I think what has happen is that the pistons had pockets for standard valves, QED fitted larger valves and these valves have made contact with the edge of the pocket. Although it could be that brief episode of over revving, I do drive the car quite hard on occasions frequently going to the red line.

I am aware of the following:

1. The engine was running well before I started this work, apart from the "tappet" noise from the worn followers.

2. Last time I checked the compression it was satisfactory.

The question I have now is what do I do? I need to remove the valves to check the follower clearance so will replace the exhaust valves. I am not sure what to do regards the pistons. I feel I have two choices, replace or machine the pockets to suit the 1.4" valves.

I don't know how accurate the valve timing is although the timing marks on the camshaft sprockets were correctly aligned at TDC (from the front cover / pulley marks).

I will try to post some photos later today. In the meantime any thoughts, criticism or advice would be welcome.

By the way is it possible to remove the pistons from below after removing the sump?

Regards

Andy

A quick note for Trevor (TeeJay) - as you will note I am busy stripping the cylinder head and would appreciate help you offered in measuring the cam buckets and sleeves in the near future.

[TeeJay]

Hi Andy.

No problem with measuring the cam buckets & sleeves just let me know when.

There was a post by Rohan on the correct procedure for replacement / setting up of valves seats/valves, shim thickness etc. If I recall correctly someone asked about reducing the length by grinding the valve stem (not recommended).

It would be useful to post photo?s to show the extent of the damage to the piston crowns. Just a thought but this could be the cause of the ?tappet? noise. But measurement & critical examination will confirm.
As you state that the compressions were OK, it appears that the valve head removed material off the pistons without bending the valve, very fortunate.

My Lotus chassis has a removable cross brace & with this removed the sump can be lowered and the pistons removed from the bottom. But depending on what else needs to be sorted, it?s easier to work on with the engine removed.

I have attached a page from the Lotus Elan +2 Workshop Manual which should help you.

I am a bit short on time today, but I?m sure you will get other informative replies.
Regards Trevor

[john.p.clegg]

Trevor,Andy et Al

I recently had my sump off in-situ and am not sure if you could remove the pistons easily,seems to me the crank would have to be dropped? therefore needing the flywheel,timimg chain and end plates etc removed ....or am I wrong ..as usual?

John

[alexblack13]

Hello guys...

No. The pistons will need removing up the way with the head removed. Just take off the big end caps and push the pistons up out of the bores. They can't be removed downwards.

Great fun that job.

I think Trevor will be right and the light contact valve / piston you have been having will be the source of your 'tappet' noise.

Enjoy!!

Alex B...

[bcmc33]

Andy,

You don't remove the pistons from below - what you do is remove the conrod bolts and push the conrod and piston up through the top. Quite easy to do as long as you have a removeable crossmember.

Let us see how much contact the valve have made with the pistons - are all the pistons showing contact?

The QED heads with 1.4" exhaust valves usually had 1.625" inlet valves - does your head? What are the cams?

New pistons are like rocking horse sh1t to get hold of these days - maybe we could machine a touch out of the pockets? Let me know.

[alexblack13]

Hi Again folks,

Why replace the pistons? just machine the pockets a bit deeper and Bob's your uncle.. Do the plasticine test to ascertain how much clearance you have and to establish how much you need to take out of the pockets. Easy peasy....

After a careful inspection for damage of course!

Saves a shed load of dosh too.

Alex..

[elansprint]

Andy whilst QED had no sight of the bottom end as you say as they are specialists in the Twin Cam i would have thought they should have asked if your pistons had been machined.
Ian

[andyhodg]

Hi Guys

Attached is a photo of No1 Piston Crown. The other are similar but I have not had chance to clean the other pistons and the photos won't give enough detail when the crowns are black.

I have filled the combustion chambers with diesel and after about 45 minutes there is no serious leakage from the valves.

Bear in mind that I did fit a 1.5 thou feeler gauge between the follower and sleeve so the clearance is definately out of spec. I don't hear the tappet noise at anything aboce 3,000 rpm. It is most noticable at idle or light throttle openings. That, to me, would indicate that it is more likely the followers than the valves touching the pistons.

Take a look at the photos.

Regards Andy

[paddy]

I've only ever seen pistons with two cut-outs in them. There were two sizes (type A and type C or something like that, from memory). Has anyone else got pistons with only one? What am I missing?

Paddy

[andyhodg]

Paddy

There are two cutouts the inlet side is the largest and the exhaust side has a small cutout with the edge of it damaged my contact with the valve.

By the way the inlet valves are 1.625" diameter. The cams are Vegantune cams which appear to be re-profiled standard cams with a lift of 0.357" from a base circle of 0.998" and timings at, inlet 105 deg ATDC, exh 110 deg BTDC. The tune is what Vegan tun called "Sprint Special" and to the best of my knowledge was done sometime in the mid 1980's

Regards

Andy

[bcmc33]

The pistons are regular Type "C" as fitted to the Big-Valve engine. The exhaust cut-out needs relieving a touch as I indicated. You could do it in-situ with a Dremel type tool, but to ensure you get the correct volume of cut you need to get the pistons on a milling machine. When you re-assemble with new rings all should be well.

[rgh0]

Hi Andy

There is certainly fairly heavy contact between the valves and pistons - I supect it is certainly due to to small cut out size not over revving. I would replace the exhaust valves as even if they have not bent, they may have suffered some damage with potential cracks in the stem just below the valve head and you dont want to drop a valve head into the combustion chamber as it makes a real mess.

The pistons may be OK and if wear is not a problem they may be reusable, they do need to be crack tested around the damged area at least. However machining the bigger pockets to get sufficent clearance may give you a problem with breaking through into the top piston ring groove, so you may need new pistons suited to the bigger cut outs.

You need to have a minimum of about 1.5mm or 60 thou clearance between the piston and valve as the piston passes through top dead centre on the exhaust stroke and the valves are partially open, the only practical way to check this without doing a dynamic CAD drawing is to do a trial assembly and plasticene test ( I use bluetac actually !). The inlet valve though not hitting the piston needs to be equally checked as it may be to close also.

Given the need to do trial assemblies you really need the engine out and on a bench to do it comfortably.

regards
Rohan

[bcmc33]

Andy,

I've just looked closely at the type "C" pistons taken from my "Busted Block" and there is clear evidence of the exhaust valves touching the cut-out at exactly the same depth point as yours. The mark is very slight but obvious when looked at through a magnifying glass. The major mark on your pistons is due to the valves being bigger in diameter.

The inlet cut-out position is closer to the periphery than the exhaust cut-out - therefore, it seems to me that extending the cut-out closer to the periphery is all that is needed to eliminate valve contact.

The cut-outs are both 3/4" radius - I have a 1" rad cutter that would do the job nicely.

Let me know what you want to do.

[TeeJay]

Andy.
Well, that will teach me to be first in with a rushed answer due to time constraints. Although I did say you would get some informative replies and you did.

Still no regrets as my wife & I did enjoy being chauffeur driven by my son, some 28 miles to a fine restaurant for an early evening meal and later for evening drinks. This was a belated birthday present; he also insisted that my wallet remained in my pocket ALL night.

So with a bit more time and knowing that there will be editorial eyes and that alcohol is still in my blood system, I will try to explain my thoughts a bit more clearly.

1) With reference to the thread that Rohan commented on. At that time I had just replaced my valve seats / guides & a 1off oversize cam follower sleeve. This thread caught my attention as it was related to valve head & interference to piston crowns. Someone had an issue with the valve lift at TDC (the amount the valve head protruded from the cylinder head face) and a suggestion was made to grind material off the top of the valve stem to reduce the amount of protrusion. This was not a correct procedure.
Rohan made several comments, including the correct selection of the thk of a cylinder head gasket. And of interest to me, the correct method of machining the valve seats to ensure correct valve lift and method of material removal to obtain max shim thickness as well as equalization of ALL the shim thk.
Now having seen your photos and the extent of the damage to the piston crowns, then Rohan comments re the valves are a must.
I assume that the pistons were not damaged before the fitting of your QED head, or that no damage resulted from setting up the static timing.

2) Yes the comments made re removing the pistons from the top with the cylinder head removed are correct. In my rush I did not fully explain. My thoughts were of lying on my aging back removing the main cap bolts etc and wishing I had the garage height for the 4 post car lift that Alex B has. No excuse though.

3) Re my comment on engine removal. I was thinking that as you previously have had the head rebuilt and you made no comment on any work on the engine block & components Then perhaps an engine out would be useful re clutch & clutch bearing, engine core plugs, ditto the water pump, checking of the main & big end bearing, etc. etc. all so much easier with the engine out.

Enough talk, I?m sure that you will consider your options carefully.

Regards Trevor
PS Have replied to your PM

[andyhodg]

Hi Guys

Thank you all for your input. I tend to feel that as Rohan suggests the problem is that the cut outs are the wrong size for the new 1.4" valves that where fitted by QED. I will get the cut outs on the piston machined to suit the large size. The damage to the pistons does show how close to the base of the cut out the valve gets so if I use that as a guide I should be able to get the correct clearance.

Brian and Trevor you both have a pm

Regards

Andy