Lotus Elan

piston cut-outs and valve clearance

PostPost by: cbguerrajr » Fri Oct 14, 2016 2:46 pm

Hi, I have a question for the engine experts.
I'm having a stock engine rebuilt and the only intake valves available are the 'big' type, so they are being fitted to the head.
I also found a set of NOS Hepolite pistons +.040" which I'm having the block bored for.
The main question is: are the cut-outs on the top of the pistons always enough to clear the big intake valves? Can any pistons, type A or C, new or old, be used with the new valves?
Also, I noticed that the step on top of the +.040" pistons is much smaller (almost flat-topped) compared to some old pistons. Is that intended to maintain the same compression rate?
I tried to attach a photo, not sure if it will work.
Any advice will be appreciated. Thanks in advance
Carlos G.
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PostPost by: gearbox » Sat Oct 15, 2016 5:58 am

Hey Carlos, I'm going through the same thing. I relined my block back to standard bores on a Europa that I bought from the original owner who told me that the engine was never touched from new. Used NOS Hepolite Power pistons and new H Beam rods at the standard length and found that the pistons now have a zero deck height. I was told it should be about 40 thou below from the factory and no less than 20 thou. But to answer your question, if everything was at factory specs, the pistons should clear with a standard gasket. Now with that said, our engines are now about 50 years old. Has the block ever been decked and by how much? Has the head ever been decked? Are you using the stock cams or a higher lift? Are the valve seats at the correct position? Has the crank ever been line bored? All these things and more can affect the piston valve clearance. So a lot of variables. The only way to see is to test out the assembled engine. Use a standard gasket with a compressed height of .027" and either clay or those plastic strips to determine valve piston clearance by turning the motor by hand to gauge if there is any interference and then checking the clay or strips to see how much they compressed. Unfortunately, you will have to assemble and disassemble the head a couple of times to do this. Depending on what you find, it may be fine, or the valves are too close. But the solution is to use the thicker Cometic gaskets to achieve the gap you want. They also come in 82 to 85mm bores to account for any oversized pistons which you are using. You will also have to cc your head chambers to determine the CR so you can dial that in as well. So a little effort and a little math and you should be ok. Good luck, Allan
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PostPost by: nmauduit » Sat Oct 15, 2016 6:37 am

Hi Carlos,

as said by Allan, you can run the calculation but a blank mount is highly recommended (compulsory to me in case mods have been made) in the end anyway...

then I would personally separate 2 issues: CR control via head gasket thickness if need be, and piston/valve clearance that call for piston top machining once the CR ratio is defined (nb: there is a limit of that exercise when one tries to increase lift a lot)

good luck and many spirited miles !
Last edited by nmauduit on Sun Oct 16, 2016 12:18 pm, edited 1 time in total.
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PostPost by: AHM » Sat Oct 15, 2016 4:31 pm

If you can get away without a re-bore Miles Wilkins has standard sized pistons at a sensible price.

By boring straight to +040 you are using all of your spare engine lives.

I wouldn't worry too much about the compression ratio, unless the head has been skimmed a lot. You can do the calculations and then ignore them!

Same with valve /piston clearance - Some people check with Plasticine.
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PostPost by: cbguerrajr » Sun Oct 16, 2016 12:39 am

thanks everybody.
the general consensus seems to be to go forward and for check clearance issues. if needed, take care of them later with a thicker head gasket. Sounds good to me.
Carlos
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PostPost by: DuncanC » Sun Oct 16, 2016 2:33 am

There is another method besides the clay method, which requires a lot of assembling and disassembling to see your results. Get hold of a dial indicator with some sort of mounting base (mine's magnetic, so I had to fab a steel plate which attached to manifold studs in order to have a place to mount it).

Here's the method: Set up the dial indicator so the moving end sits on top of the spring retainer with the plunger axis parallel to that of the valve stem. Rotate the engine to TDC and start checking clearance every 2 degrees or so by forcing the valve farther open until it touches the top of the piston. The change in the measurement on the dial indicator between the "rest" position and when the valve touches the piston is your piston-to-valve clearance. (Some suggest doing this at zero lash setting, which is obviously easier said than done on some engines; doing so does add a safety factor). Expect the smallest clearance to be around 10-12 degrees after top dead center. Some will say 0.080" is the minimum acceptable figure where others say 0.050" is fine. I kept mine at 0.060 on a 1200 cc NSU overhead cam hemi-head race engine and it revs to 7000+ with no issues.
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PostPost by: DuncanC » Sun Oct 16, 2016 2:38 am

There is a downside to "taking care of it later with gasket thickness." Ideal squish area height is 0.040" and this minimizes the chance of detonation. If your pistons are flush with the block deck you can use a 0.040" gasket height (after torquing) to obtain this. But you will have had to have already calculated your compression ratio, which needs to match your camshaft grind.
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PostPost by: CBUEB1771 » Sun Oct 16, 2016 3:31 am

DuncanC wrote:There is another method besides the clay method, which requires a lot of assembling and disassembling to see your results. Get hold of a dial indicator with some sort of mounting base (mine's magnetic, so I had to fab a steel plate which attached to manifold studs in order to have a place to mount it).

Here's the method: Set up the dial indicator so the moving end sits on top of the spring retainer with the plunger axis parallel to that of the valve stem. Rotate the engine to TDC and start checking clearance every 2 degrees or so by forcing the valve farther open until it touches the top of the piston


This is also my preferred method. I do a trial assembly with very light springs instead of normal valve spring sets. This allows me to push a valve all the way to a point of interference with very little force.
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