Long rods, short block...

[Davidb]

Hello all, I am sure some will roll their eyes at the above title-"Oh Jeez, not again!" Yes, I am afraid so. I did do a search on the forum and found the following thread lotus-twincam-f39/twin-cam-build-t25127.html
I have emailed Carl who started the thread and asked him for his experiences.
My situation is that I have to rebuild the engine in my old racing Elan after the previous owner froze the block and wrote it off having already destroyed the steel crank and other parts.
I have a good block-bored to 84 mm and I was just given a Formula Atlantic BDD crank-I am not going to race the car but I am going to use it in historic rallies-in North America-where the last time I checked nobody cared if you had 1700cc just so long as your entry payment cheque cleared the bank.

So, any suggestions as to crank/rod/piston combinations?
Here are some that I am considering:
http://www.ebay.com/itm/151042370620?_t ... EBIDX%3AIT

http://www.ebay.com/itm/151670407989?_t ... EBIDX%3AIT

I don't think the pistons can work due to compression height--(too high by 3.4mm according to my terrible arithmetic)

I found a mention that Datsun 240Z pistons can be made to work and as a result of learning that I contacted Rohan-hopefully he will chime in here-he seems to be one of the most experienced Twin Cam builders here. Also, as an Australian he views things differently, in my experience. Aussies have to make things work with what they have, not what the rest of the world might have...a different way of looking at things!

Thanks, and Merry Christmas to all

[Davidb]

In reply to my own thread-I did a lot more reading and I think I have decided that short rods are the way to go with the short block--I heard back from Carl who started the thread I referenced above and although he has had the parts made other projects got in the way so he has nothing to report at present.

Original Datsun 240Z pistons are available and cheap but as Rohan told me in an email they are heavy-about 400 grams I believe compared to about 300 grams for a modern forged piston. I am hoping to find some forged 240Z pistons that are .040" over that will give me 84mm.

Why not just order from CP or Arias or Dave Bean or QED I hear some ask? Well, here in Canada our dollar was on equal footing with the US dollar for most of the past ten years, then, about a year ago, it started to drop, in the past few weeks it has plummeted to the point where we are now 40% below the US dollar! That makes buying in the US and the UK very expensive--hence my search for deals!

[rgh0]

The Datsun L series pistons from either the 240Z or 1600/510 can be used but I have never done it myself so I don't know the full details of whats required. The standard Datsun L pistons have a shallow bowl in them of about 7 mm deep. This needs to be machined off to give a flat top and then valve cutouts machined. I am not sure of all the dimension details that you need to aim for in a short block and long stroke crank engine but I believe the compression height on the modified pistons and using either BDD rods or standard 1600 rod length ( not sure which) gets you near right. I believe you need to be careful with the depth and position of the valve cutouts to avoid the top ring groove.

I know a couple of people who have built these engines and will followup on what they did

cheers
Rohan

[Davidb]

Thanks Rohan, The Datsun 510 pistons do have a dish but the 240/260Z pistons are flat topped I believe which should help.
Looking forward to hearing from your friend who has done this. I think I have seen posts by him before.

[elanman999]

Get JE pistons to make exactly what you want. Yes they are in the US but ring Jon Savage at Cambridge Motor sport, he'll get them ordered. I have not connection other than a satisfied customer.
Or some of the Accralite pistons from Burtons can be modified to fit.
Edit, I should explain, Some of the Accralite pistons have an unfinished crown so they can be machined to your choice of length, shape etc.
Cheers
John

[holywood3645]

JE or CP pistons are both great quality items, They will make them to any spec you need.
James

[oldchieft]

This what I wrote in a post a while back on the subject of rod length and stroke. My engine is 122/84, or 1.45.

Quote.
Rod to Stroke ratio affects a lot of things (low can be 1.45 to 1.75), 122/84 is 1.45, so this is the low end of low.

This is the ratio for the Small Block Chevy 400 with std. rod, so it is not so extreme as to be beyond main stream manufacturing.

Low ratios are good at low down power due to the faster acceleration of the piston and the better gas velocity at the start of the induction stroke also better squish turbulence on compression, with my bodge set up both these work well for me!

Also earlier timing (more advance) is required, as the chamber volume is larger (piston is farther from TDC) at the same point of rotation.

On the down side at high revs the piston can out pace the flame front.

A higher level of vibration, my balance man back in the day was one of best in the game. (I hope I can still find him)

High piston side load giving rise to wear and higher heat load, so short piston skirts and high revs are not good.

I did not know all this when I built the engine, so dumb luck played a big part in building an engine that suited what I wanted. I love low pulling power for relaxed cruising.

For what it is worth that is my take on short rods, if anyone wants to put the other side I would be glad to hear it.
Un-quote.

If you are not looking for screaming revs and have a piston with a good depth of skirt then short rods are fine, at lest they have worked for me.

also you may wish to look at: -

http://victorylibrary.com/mopar/rod-tech-c.htm

Jon the Chief

[europatek]

Hi David, you may be interested in this, my last twink rebuild;
lotus-twincam-f39/twincam-bore-size-and-pistons-t27689.html
I've used Datsun pistons a number of times. Using an L block, 711m 1600 crank, 125E rods. The Pistons are then modified to suit which is pretty easy.

[Davidb]

Thanks for the responses! For those who are wondering why I am not using established piston manufacturers the answer is in my second post above.

Oldchief: Thanks for the input. The whole idea of boring and stroking is so we don't have to rev the nuts off it to get decent power isn't it? So I think you have talked me into short rods.

europatek: Thanks for your info-it was you I was thinking of who had done this before with Datsun pistons. They appear to be original stock, cast pistons-is that the case? Rohan was concerned about their weight-did you manage to lighten them?
What modifications did you have to do to use them?
Many thanks.

[europatek]

Hi David, the pistons I've used have all been cast replacements from ACL or similar and for a road engine have proved more than satisfactory. I can't remember the final weight but they end up slightly lighter due to the mods needed. The mods needed are;
- flycuts for valve clearance
- gudgeon pins shortened and circlip gooves added (makes the pin fully floating as opposed to Datsun press fit)
- little end bush reamed to suit the pin
- piston height reduced to sit flush with block deck
- balanced
Hope this helps.

[Davidb]

Europatek: Have you ever determined the compression ratio following these modifications?

[europatek]

I did years ago when I first built the engine and from memory it was around 10.2:1.

[rgh0]

I did years ago when I first built the engine and from memory it was around 10.2:1.

Thanks for helping out.... saved me giving you or Richard a call. From the piston dimensions it looks like you could use a 4.928 inch 1600 length rod with a machined down low compression L piston with the depression in the top as well as your approach of a 4.800 inch 125E rod with the flat top higher compression piston. Do you know anyone who has tried the longer rods in a short block?

cheers
Rohan

[europatek]

I doubt you'll be able to use the long rod with a Datsun piston as you'll need to remove an additional 3mm from the crown to make the comp height correct and that will be too much off the crown. Special pistons for that I reckon.

[rgh0]

The low compression pistons that appear to have been used in the US Datsun 510 had a 4 mm recess in the top - Machining of the lip of this recess gives you a flat piston with a 4 mm shorter compression height. Which should fit nicely with the longer 1600 rod. The required valve cutouts may be an issue depending on exactly where the top ring groove is?

cheers
Rohan