Lotus Elan

Accralite pistons' unique top shape

PostPost by: groucho » Fri Apr 27, 2018 3:13 pm

So I had to pull my engine to replace the water pump, so I figured that I should get the bottom end gone over (the head was recently done.) The machine shop found that the pistons were a little loose in the bores and recommended larger pistons. We determined the +0.030" over pistons would be an appropriate size. I tried to order QED's forged pistons in that size but they were unavailable for a month or so. Burton also showed unavailable, so I decided to spend a little more and get the Accralite 83.25mm pistons which are very close to that size and should fit the engine nicely. http://www.burtonpower.com/accralite-fo ... fl111.html

So the pistons arrived and I hadn't noticed in the web photo, but they don't have the usual twin cam flat-top with valve cut-outs; there's sort of a recessed dome and they are slightly shorter than the stock pistons (well, whatever pistons I had before.) See attached image for the actual pistons that I received.

I don't see any mention of this top design in the descriptions anywhere and can't find much info on the internet about it; I'm assuming the pistons will work fine (the bore size is certainly fine); maybe with a higher compression? I'm really hoping that they'll be fine, they were certainly not cheap!
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PostPost by: avro » Fri Apr 27, 2018 4:54 pm

Hi

The pistons that you have purchased are a forged Competition piston with a high intruder giving a 11.8 CR as opposed to the forged Road piston that is copy dimensionally of the original cast piston (9.5CR) that is in short supply. The pistons you have are slightly shorter but that is only in their overall height , not in the compression height so they will sit in the cylinder block in the same position at TDC.
The high intruder on the pistons you have purchased when used on a race engine would normally require machining to achieve a suitable compression ratio for a particular application so this could be done.
However they do have larger and deeper valve pockets than the standard piston and therefore this will need to be taken into account when working out your compression ratio and machining the piston as required.
If your block has already been rebored at +30 don?t be misled into thinking that these pistons are the equivalent size. They are in fact .0025? thou smaller and if fitted will give you far too much bore clearance.
Perhaps it would be better to return these pistons and wait until the size you require is available or opt for a larger oversize.

Nick
Last edited by avro on Fri Apr 27, 2018 9:31 pm, edited 2 times in total.
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PostPost by: groucho » Fri Apr 27, 2018 5:05 pm

Thanks for the reply, Nick. I wondered if the actual height of the compressed area was identical.

The machine shop has bored my block but only after receiving these pistons and boring to match them. It was understood that they were not exactly +0.030" but they were near enough to require only a minimal boring to fit; I don't want to take out any more than necessary as I want this engine to last a long time.

The engine is going in a purely street-driven car and has a more or less stock head; I'm all for extra power but I am most concerned with a reliable and well-running engine. Should I expect to be OK with the higher compression with regular gas? Quick Google research indicates that the answer is "no"!

Do I have any options to bring down the compression to a reasonable level; in case the machine shop has already installed the pistons? I'm not so sure that Burton would be interested in me returning pistons that have been installed already.....
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PostPost by: groucho » Fri Apr 27, 2018 5:53 pm

OK, so I verified that my machine shop has not done anything with the pistons so I've emailed Burton about returning them and hopefully the +0.030" pistons will be in stock soon. The machine shop is grumbling about having to take the crank back out and rebore the block for larger pistons again but what can you do...

It's all a bit frustrating; it's probably going to cost a fortune to ship the Accralites back to the UK from the US, but hopefully I'll recover some by going to the cheaper Omega forged pistons.
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PostPost by: Foxie » Fri Apr 27, 2018 7:59 pm

groucho wrote:OK, so I verified that my machine shop has not done anything with the pistons so I've emailed Burton about returning them and hopefully the +0.030" pistons will be in stock soon. The machine shop is grumbling about having to take the crank back out and rebore the block for larger pistons again but what can you do...

It's all a bit frustrating; it's probably going to cost a fortune to ship the Accralites back to the UK from the US, but hopefully I'll recover some by going to the cheaper Omega forged pistons.


If the block has been bored to suit the pistons, then one way to go would be to have the dome machined off. I've done this many times when using these type of pistons. You need to calculate the volume of the dome needed to get a specific compression, or assemble the engine and measure the combustion chamber volume and take it from there. I've found that when the dome is machined off level with the piston top you are very much in the ballpark with the standard engine capacity.

BTW, I got my last set of pistons from Dave Bean, San Andreas, CA. :)
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PostPost by: nmauduit » Fri Apr 27, 2018 9:10 pm

if you're set to use these high quality pistons, a machine shop can machine off a bit of the intruders so that they don't raise the compression ratio more than what you want them to exactly. A bit extra work but you'll have everything at hand to get going...
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PostPost by: groucho » Fri Apr 27, 2018 9:25 pm

My only concern with machining these is that the shop (like most shops especially here in the US) is not super familiar with the Lotus twin cam. They do seem competent but they are only working on the bottom end of the engine and are not interested in doing any additional assembly work. Would they need the head to know how much to machine off? I understand that a thicker head gasket can help too but I don't know enough about the specifics to know what's involved with them knowing how much to remove.

They did have to deck the block slightly, too, so there's that, too.

I think that if Burton will take the pistons back, the cheaper regular compression pistons should pay for the return shipping, and then there's no concern about machining very expensive pistons.

I don't think I've ever bought anything from Dave Bean for whatever reason, probably a combination of no online catalog, and RD being much closer.
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PostPost by: Elan45 » Sat Apr 28, 2018 2:11 pm

You're right around the corner from Tony V. If he's got the time to help, I'm sure he could offer some good advice and perhaps a shop that is more familiar with the twin cam engine.

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PostPost by: groucho » Sat Apr 28, 2018 2:28 pm

Elan45 wrote:You're right around the corner from Tony V. If he's got the time to help, I'm sure he could offer some good advice and perhaps a shop that is more familiar with the twin cam engine.

Roger

I know Tony; in fact, this Elan was his first Lotus, and he's certainly offered to help but he's about an hour away so it's a bit far for this type of thing. His preferred local Lotus machine shop closed up in 2016 - my engine was there at the time for the bottom end work; they disassembled it, did a little cleanup on it, they called and said to come pick it up and that I should be able to rebuild it. So instead of a rebuilt bottom end, I just got a pile of parts. Not exactly what I was hoping for... it sat since then until I decided that I really didn't want to go through reassembly on my own and took it to this other shop where it is now. The hope is to get it together in time for LOG this year. There's a lot of other stuff I need to do, too; that's one of the reasons I was trying to push off the engine work on the professionals, so I could focus on other things.
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PostPost by: nmauduit » Sat Apr 28, 2018 3:33 pm

groucho wrote:My only concern with machining these is that the shop (like most shops especially here in the US) is not super familiar with the Lotus twin cam. They do seem competent but they are only working on the bottom end of the engine and are not interested in doing any additional assembly work. Would they need the head to know how much to machine off? I understand that a thicker head gasket can help too but I don't know enough about the specifics to know what's involved with them knowing how much to remove.

They did have to deck the block slightly, too, so there's that, too.

I think that if Burton will take the pistons back, the cheaper regular compression pistons should pay for the return shipping, and then there's no concern about machining very expensive pistons.

I don't think I've ever bought anything from Dave Bean for whatever reason, probably a combination of no online catalog, and RD being much closer.



you don't need to be super precise for the CR ratio, just to be in the ball park for use with your local octane ratio.

From the link you gave I see

Apex height: 6.0mm
Apex cc: 8.7cc

doing just an approximation you could calculate the amount of apex you need for say a Sprint CR of 10.5:1 (say 1mm compressed height gasket, regular 72.75 stroke, 37.5cm3 chambers 0.2mm block gap clearance below deck) i.e. about 2.5cm3, and do a linear approximation 6mm / 8.7c * 2.5cc = 1.7mm

so machine the apex off until 1.7mm remains out of the initial 6mm (i.e. take out about 4.3mm off the intruder)

(please double check with your actual engine specs, or your machine shop can also do that for you in 2 minutes)
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PostPost by: groucho » Sat Apr 28, 2018 4:59 pm

Thanks for the math. It's all greek to me :lol: but hopefully makes sense to the machine shop.

I know the machine shop is anxious to be done with this so they can get back to their usual Chevy V8s and other boring engines! So I can present this to them as an option. I know they'd rather not have to bore out the block again (and I'd rather not pay for them to.)
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