A piston query!!

PostPost by: AussieJohn » Mon Jan 16, 2012 3:49 pm

Did the sprint engines have pistons with larger valve cutouts than standard [ i.e. like powermax pistons] or the small cutouts like earlier engines?
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PostPost by: bcmc33 » Mon Jan 16, 2012 8:49 pm

AussieJohn wrote:Did the sprint engines have pistons with larger valve cutouts than standard [ i.e. like powermax pistons] or the small cutouts like earlier engines?

No. From my experience the early pistons had large cut-outs and the big-valve had small cut-outs
Also from my experience, (AndyHodg really), when you use the larger valves in a big-valve head, the exhaust valve shaves the outer wall of the cut-out.
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PostPost by: AussieJohn » Mon Jan 16, 2012 9:24 pm

Hi Brian, from my own experience the early engines had the small cutouts but I have never had an original sprint engine so I am in the dark; I have a problem with a standard sprint engine but with +.020 small cutout pistons. The inlet valves are just touching the bottom of the cutout; the valves and seats are new the shims are circa 112thou; the cams are standard sprint and the pistons are what was in and running OK. Where is my problem? cheers, John.
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PostPost by: bcmc33 » Tue Jan 17, 2012 1:02 am

AussieJohn wrote:Hi Brian, from my own experience the early engines had the small cutouts but I have never had an original sprint engine so I am in the dark; I have a problem with a standard sprint engine but with +.020 small cutout pistons. The inlet valves are just touching the bottom of the cutout; the valves and seats are new the shims are circa 112thou; the cams are standard sprint and the pistons are what was in and running OK. Where is my problem? cheers, John.

Appears like timing to me, John.
From memory, Andy's engine had QED360 cams that were well off the bottom of the cut-outs.
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PostPost by: alan.barker » Tue Jan 17, 2012 7:43 am

for me the early pistons are large cut outs and later small cut outs. I hope the pistons are facing the correct direction (i know silly question but!)! Both my Sprint and +2/130s are on standard size bores and small cut out pistons :mrgreen:
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PostPost by: AHM » Tue Jan 17, 2012 12:44 pm

alan.barker wrote:I hope the pistons are facing the correct direction (i know silly question but!)!

...Because the cuttouts are different sizes for inlet and exhaust

The earlier piston also has a raised crown so the cutouts are even larger
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PostPost by: andyhodg » Tue Jan 17, 2012 12:57 pm

Hi

As bcmc33 said I had problems with the QED exhaust valves just touching the side of the pocket, as you can see in the photo. With a 360 lift cam there was plenty of clearance with regard to the depth of the cutout but the radius was marginal. This contact hadnot damaged the valves (although I replaced them for safety's sake.

As you will see there is no problems with the larger inlet valve pocket.

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PostPost by: john.p.clegg » Tue Jan 17, 2012 1:51 pm

Andy

I'm no guru but surely metal to metal contact (valve to piston) is not best practice??

John :wink:
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PostPost by: AussieJohn » Tue Jan 17, 2012 2:45 pm

Thanks for all the tips guys, hoping for success now!
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PostPost by: bcmc33 » Tue Jan 17, 2012 3:36 pm

john.p.clegg wrote:Andy

I'm no guru but surely metal to metal contact (valve to piston) is not best practice??

John :wink:

John,

If you're referring to Andy's problem, then you're quite correct. That's why Andy was concerned and had me machine the pocket wall for clearance.
What seemed odd to me at the time was that the edge of the inlet pocket to the OD was less that that of the exhaust pocket. I machined the exhaust side to the same dimension as the inlet and it eliminated the evidence of the foul condition you can see in Andy's pictures.
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PostPost by: Jeff@Jae » Tue Jan 17, 2012 7:48 pm

Wow, I haven't seen a cast piston used in a Twin Cam in years ! Marginal valve reliefs, ancient technology piston rings, soft ring lands etc. have ruled those out a long time ago. If valve clearance is an issue them fly cutting the valve pockets is the best way to go for sure.
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PostPost by: AHM » Tue Jan 17, 2012 8:55 pm

AussieJohn wrote: Where is my problem?


Interesting pictures! I?m intrigued; I can only think that to move the contact point out towards the piston circumference you would need bigger valves, to change the angle of the valves, or skim the head. (edit: and timing as mentioned above)

Jeff@Jae wrote:Wow, ... ancient technology


That sums-up most aspects of the ?lan design.

I?m just about to put a set of cast pistons in my S3. They seem quite adequate for road use and it would take a while to off-set the price difference.

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PostPost by: Jeff@Jae » Mon Jan 23, 2012 4:29 pm

Good point Simon and that certainly has to be understood when dealing with our Elans. However, we can utilize newer technology here and there. Things like forged pistons with contemporary ring designs, better valve guide and valve seat materials, lighter gear reduction starters, for example, don't change the character of the car but simply make them more efficient/last longer. I usually don't advocate trying to make a modern car out of an Elan because it generally ruins the car and if a modern sports car is what one wants it's easier to buy an Elise. The exception would be trying to build an Elan for racing and even then rules prevent going contemporary in a lot of areas, especially in vintage racing. My S2 Elan has some of the above mods but it's an honest S2 Elan and a great car the way it is, not a wannabe Cobra.
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PostPost by: StressCraxx » Tue Jan 24, 2012 3:06 am

Sorry Jeff, I'm still running the same set of Hepolite +.020" pistons you sold me in 1983!

Yes the ring lands are beat up, but its amazing how forgiving the Deves piston rings have been at keeping good compression and the oil in the sump.

Amazing what clean oil & air, and keeping it spun to 7K on a regular basis will do.

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Dan
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PostPost by: Jeff@Jae » Tue Jan 24, 2012 4:44 pm

You hit it right on the head Dan. Regular, spirited use and good maintenance is the key to a happy Elan. Mine thrives on it ! Too many of these cars spend more time sitting around being "improved" instead of being enjoyed.
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