Lotus Elan

Enlighten me about con rods?

PostPost by: vxah » Tue Apr 02, 2019 3:41 pm

Can someone explain why the twincam rod length is reported as being 4.80" and so many aftermarket rods listed for the twink are listed as 4.826"?
Is the oe length just written as 4.8" to save ink or are the aftermarket ones really longer and, if so why?
I am kind of hoping they really are 0.026" longer so this way I can use a set to raise my pistons above deck height to gain some compression, I can machine down the outside of the crown 0.026" to produce a slightly domed piston? They already have larger valve pockets and a very slight dome.
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PostPost by: promotor » Tue Apr 02, 2019 3:50 pm

The factory twin cam rod is 4.800" - some aftermarket ones are 4.826" and as you suspect it is to raise the piston higher in the bore.
I have some steel Cosworth rods that are 4.826" yet I have some steel Farndon rods that are 4.800" - all depends on what is required and whether there is room at the top of the bore.
I guess it was just easier to use longer rods - if building a new race engine etc with new con rods - rather than skimming the block.
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PostPost by: vxah » Tue Apr 02, 2019 4:25 pm

I have a 681F block with steel caps and forged crankshaft, the pistons are quite new and the block has been decked to bring the pistons pretty much flush, It's built with reworked 125E rods that perhaps are not as good as aftermarket ones nowadays, even with ARP bolts?
My cylinder head has been tweaked and working things out I am struggling to better 9:1 static compression ratio, I don't want to skim the head if I can help it or buy new pistons so changing what might be a weak link and some machining work on the pistons could go some way to help? Maybe use a thin MLS head gasket?
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PostPost by: promotor » Tue Apr 02, 2019 4:58 pm

vxah wrote:I have a 681F block with steel caps and forged crankshaft, the pistons are quite new and the block has been decked to bring the pistons pretty much flush, It's built with reworked 125E rods that perhaps are not as good as aftermarket ones nowadays, even with ARP bolts?
My cylinder head has been tweaked and working things out I am struggling to better 9:1 static compression ratio, I don't want to skim the head if I can help it or buy new pistons so changing what might be a weak link and some machining work on the pistons could go some way to help? Maybe use a thin MLS head gasket?


I'm going through this myself at the moment - I have a fairly low compression ratio on one of my engines so I'm thinking of skimming the head slightly, and it's likely I am going to use a 1600 crank and some slight intruders on the pistons. Also thinking about some forged 85mm pistons which I have sat waiting that are flat top and suitable for machining to the required shape for cutouts and intruder.
For a road engine I would want no less than 0.040" between the piston outer edges (ie outside of intruder) and the cylinder head.
I think the 125e rods are very good rods - 125e rods are SDF rods (Smethwick Drop Forged) which is the same maker as my Cosworth rods. If you are concerned about them I would get them crack tested. That said it can't hurt to buy new good quality steel rods if it helps with the numbers.
I think you'll need to find approx 8cc's in your pistons/intruder/gasket etc to go from 9.0:1 to 10.5:1 in a 1594cc twincam by my calculations. That's a fair amount!
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PostPost by: Davidb » Wed Apr 03, 2019 5:20 pm

I bought a set of the "MaxSpeeding" rods on ebay. These closely resemble Carrillo rods but are 1/4 the price. They are 4.826". My engine machinist (who builds 4500 bhp dragster engines as a matter of course) was very impressed with them-unlike the American made rods I bought for my last engine. I searched the intraweb and could not find a negative report on them. However, when I ordered pistons from a Twin Cam specialist they failed to take the extra length into consideration (despite being advised several times) and I now have to use a thick head gasket! So be careful.
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PostPost by: vxah » Thu Apr 04, 2019 7:33 am

Yes I have seen the Maxspeeding rods and thought, I could not use them just because of the name! However, they would give me the result I require.. There was a narrow journal set on ebay for ?120! Maybe I could buff off the writing?
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PostPost by: rgh0 » Thu Apr 04, 2019 9:07 am

The 4.826 rods were introduced by Cosworth as far as I am aware ( but maybe it was BRM) to increase the compression ratio and reduce the squish volume in the early Twincam racing engines. Why they did that versus changing the piston compression height and intruder design I don't know but I presume it related to either what was allowed under some racing regulations of the day or perhaps related to piston manufacturing technology of the day limiting what piston changes were economically possible

Other makers of racing rods followed suit and this continues today. It was fine back then when building race engines on new blocks but can become more challenging when building engines on old blocks (and heads) that have had a lot of skimming.

The Maxspeeding rods appear to work pretty well in racing engines and i know people who have used them without problem. If I used them I would try to get them without the claimed ARP bolts and source my own bolts as these bolts are the most critical component in the engine and I would not trust a rod bolt out of China that claims to be an ARP bolt. Having said that i have enough spare sets Carrillo rods to have not needed new ones in a while and have not personally bought Maxspeeding rods and I would have to think long and hard before I trusted an expensive 8500 / 9000 rpm race engine to them.

If building a road engine, especially if trying to push the rev limit to 7000 / 7500 rpm and needing new rods the Maxspeeding rods are about as cheap as any new rod option so i would probably use them in that application.

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PostPost by: vxah » Thu Apr 04, 2019 10:26 am

Thanks Rohan, it would be a road engine and not exceeding 7500 rpm so I may look into those rods. I was also thinking about an offset grind to the crank as the big ends need a clean up. That way I would gain a few cc's as well. My machine shop guy says he won't do it due to the set up time etc but he could send it to be done.. Would this cause balance issues?
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PostPost by: 2cams70 » Thu Apr 04, 2019 11:06 am

You'll need to reduce the overall diameter of the big ends more so than necessary for a standard stroke in that case. You may wish to consider the reduced strength and life of the crankshaft and whether there are undersize bearings available to fit after it's been offset ground.
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PostPost by: rgh0 » Thu Apr 04, 2019 11:22 am

If its a standard ford crank then 7000 rpm is a real hard limit, Seen a lot of broken cranks and blown engines from people who try a few hundred rpm more. Offset grinding to increase the stroke a little should not cause a balance problem or strength problem in a road engine but you do need the suitable bearings to be available. It may however cause some clearance problems with the head and valves especially with longer rods and depending on the block, and head skimming and pistons and head gasket thickness used.

Lots of possible variables so you need to calculate the end clearances with any "modified" or "as new original " build.

I am currently trying to convince a large multi-national high speed machine manufacturer that their million dollar machine has not been dimensioned or toleranced right to get the correct clearances. So far with little success despite the interference marks being blatantly obvious on a trial rebuild following failures of two of the original factory supplied assemblies in service.

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PostPost by: Davidb » Thu Apr 04, 2019 3:37 pm

My understanding is that the Maxspeeding rods are made in China but machined in California where the ARP rod bolts are fitted. Certainly , my machinist was impressed with the machining when he torqued up the rod bolts and then measured all the rods carefully. My previous set of rods, made by Cunningham in the USA failed this test.
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PostPost by: nmauduit » Thu Apr 04, 2019 4:00 pm

Davidb wrote:My understanding is that the Maxspeeding rods are made in China but machined in California where the ARP rod bolts are fitted. Certainly , my machinist was impressed with the machining when he torqued up the rod bolts and then measured all the rods carefully. My previous set of rods, made by Cunningham in the USA failed this test.


I'm not sure, I got a set some time ago and it was shipped from China (haven't got a chance to open the wrappings to check if bolts were legit Arp or fakes - not sure I would be able to see - looks like I'm not going to use them soon now that most my parts for that build are impounded or otherwise stuck following the discovery of dubious activity at MKF Engineering)...
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PostPost by: vxah » Thu Apr 04, 2019 4:35 pm

I have been doing more measuring and thinking! Might be best to just buy a set of intruder pistons to get the compression I want but, if I do that the 4.826" rods are not going to work because my piston to deck clearance is now at about 0.003" on the worked over 4.80 rods.. I don't want to be machining the new pistons to clear the head or using an extra thick gasket.
that said, if the 125E rods are reliable then I can use them, they appear to have been balanced , shot blasted and fitted with ARP 2000 bolts.
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PostPost by: Davidb » Thu Apr 04, 2019 5:09 pm

I had a quick google and found this:
https://www.benzinga.com/14/09/4843601/ ... auto-parts

Which says that their ARP rod bolts are made by a Chinese manufacturer...

In any event, I still cannot find a negative report on them.

Edit: These rods are available without rod bolts supplied-the rod bolts could be acquired from ARP directly if that makes the buyer happier.
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PostPost by: 2cams70 » Thu Apr 04, 2019 11:02 pm

If you are intending to use a standard cast crankshaft there's definitely no point in going to aftermarket steel rods. The crankshaft and round main bearing caps (if fitted) are the weakest link in a LTC. I certainly would never consider offset grinding the standard crank and weakening it even further. Even in standard road going engines the crankshaft when professionally crack tested often shows up as being cracked. You'd be much better spending your money on crank and caps. QED have a reasonably priced steel crank on offer made from EN19 steel.

Sorry I see you already have steel caps and forged crank. Steel rods are worthwhile then. I still would not recommend offset grinding an expensive steel crank however. The performance gains (if any) from increasing the stroke would be the size of a bee's dick. I had the same problem with my engine. Pistons flush with block and off the shelf steel rods 4.826" long. I had a set of rods made by Saenz in Argentina to the standard 4.800". Very good quality and reasonably priced - Saenz are suppliers to Cosworth these days. Note there's a set of used Arrow steel 4.800" rods on Ebay UK at the moment. Make sure you specify the correct journal width to suit your crankshaft - wide or narrow.
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