Why 4.826" conrods?

[2cams70]

Hi Guys,

I'm new to this forum but have been doing a lot of reading of the various posts and have found it a wealth of information and advice. Thanks for that.

I'm currently in the process of putting a Twin Cam Escort back on the road after it's been sitting for 17 years (Sorry guys, hope it's ok that it's not an Elan - but I do like those too!). My midlife crisis project - so looking forward to hearing that wonderful engine again before I die. I've missed it greatly.

First step has been getting the engine back into fine fettle. I've rebuilt quite a few crossflows over the years but this is my first attempt at a Lotus Twin Cam. I was originally just going to do a standard rebuild but upon inspection the original crankshaft was found to be cracked. At that point I decided to buy a steel replacement crankshaft from the UK, which led to steel main caps, steel conrods, QED 420 cams, full head rebuild with sprint valves, etc. In other words going well down the slippery slope!

Upon doing a dummy assembly of the bottom end using the standard 125E connecting rods I find that the squish band of the pistons is practically level with the top of the block. Pistons are +060 standard compression Omega forged type (regrettably the block was already +040 but sonic testing suggested block ok to be bored +060). Looking at available aftermarket rods from various suppliers I find that the standard length quoted is 4.826" and not the 4.800" of the standard Lotus rods. I understand in a virgin engine that hasn't been decked or line bored the extra length is designed to bring the pistons up level with the top of the block but why play around with the conrods? why not just deck the block .026"?!! You can't get crossflow or BDA conrods just 0.026" longer than standard so why the Lotus Twin Cam? I know that there are some theories about longer rods resulting in less frictional losses but I can't imagine just .026" making much of a difference. There must be some history as to the development of the 4.826" rod length and I'm sure it's somehow a throwback to the early Cosworth days. If anyone knows the history I'd love to know. Come to think of it if anyone knows the development history of the narrow journal crankshaft and 12 bolt flywheel I'd like to know that too! (yes I really am weird).

I know I'm rambling so the end result is that I have to make a decision whether to order 4.800" rods as a special (long lead time, maybe extra cost) or machine down the pistons around .026" or perhaps obtain another set of pistons. I've crunched some numbers and with the pistons proud of the block I can't get a reasonable 10 - 10.5 CR for road use fuel using a head gasket that is not thick MLS. I don't want to use an MLS gasket based on what I've read here. Combustion chamber volume measures as 36.7cc and piston crown volume around 1.2cc. I spoke to the piston manufacturer and they ummed and ahhed a bit about machining the pistons that amount. There doesn't seem to be as much meat in the crown to play around with in the standard compression type piston compared to the high compression type. In hindsight maybe I should have bought this type instead but it was only after buying the pistons that things started getting out of control (thanks PayPal and Mastercard) and the block is already bored to size now. In anyone's experience would the valve pockets need deepening or can just the squish band and raised centre section of the piston be skimmed? Looking at the piston it seems that the bottom of the valve pockets run very close to the top compression ring so I don't think there's much scope there for any machining

If anyone could offer any suggestions or further information it would be most welcome.

Paul

[dougal9887]

If you relieve the shrouding around the valves, you will be suprised at the increase in combustion chamber volume, I was! Ended up having to use a thin MLS gasket to gain compression despite pistons being only 17 thou below deck.
Dougal.

[rgh0]

I have always assumed like you that the 4.826 rods were developed historically for a higher compression ratio and maybe also suited an existing forging from some other application or maybe was somehow dictated by racing rules of they day. I have always used these length rods from Carillo and selected / machined pistons to suit deck heights I had and compression ratios I wanted

The narrow journals I have assumed came from BDA / DFV rod and crank development that was applied back to the Twin-Cam as an option for racing engines and cranks.

I don't know much about omega standard compression forged pistons but other forged pistons I have looked at have plenty of crown thickness for what you want to do. You will probably need deeper than standard valve pockets to accommodate the higher lift longer overlap QED 420 cam if you just have standard pistons cut outs. Again most forged pistons can accommodate these. I have used a range of pistons over the years but tend to use JE pistons these days. I have seen some pistons where the pockets have broken through into the top ring groove so you do need to be careful

regards
Rohan

[Chancer]

I didnt think that rod length changed the compression ratio?

Maybe the rod length came about through building a short stroke screamer engine to reach the maximum displacement within race regs?

And the narrow journal cranks just used the standard pre X/flow big end bearings didnt they?

[rgh0]

I didnt think that rod length changed the compression ratio?

Maybe the rod length came about through building a short stroke screamer engine to reach the maximum displacement within race regs?

And the narrow journal cranks just used the standard pre X/flow big end bearings didnt they?

The slightly longer rod pushed the top of the piston at TDC closer to the head making it easier to build a high compression engine.

The rods widths on all the standard Ford 1500 / 1600 kent blocks from the early 60's 4 bolt cranks 1500 to the last mid 70's 6 bolt crank 1600 were all the same. The narrow journal crank and rods appears to have been a racing development around 1966/7 at the time of the BDA / FVA / DFV development to reduce drag and potentially to fit the rods into the DFV V8

regards
Rohan

[Chancer]

Yes, re the compression, of course it reduces the unswept volume, I was having a senior moment.

[2cams70]

Thanks guys for your responses - especially Rohan

Does anyone have any opinions on how the racing type pistons and rings stand up in normal road use? Eg. do they slap around when cold, use oil and wear out fast? Speaking with QED and Omega the standard type and high compression type pistons use the same forgings. The standard piston has wider rings and a 2 piece oil control ring compared to 3 piece on the high compression (racing) type. The heads are machined differently of course for application specific valve and compression ratios. Reading snippets of information on the internet it seems there is quite a bit of science that goes into piston and ring specification for any given application. Investigated some Cosworth pistons but found out that piston to bore clearance recommendation was .008". I can't imagine that translating well for normal road use.

I must admit I find the Ford Kent engine family history fascinating - full of mythology like weirdo engine block and casting numbers, 4.826" rods, etc. Wonder what the original designers would think if they were still around? Even a 500+HP BDT-E turbo engine can trace it's family tree back to the Kent's first incarnation as a humble 39HP 997cc Anglia motor. Can't really think of any other engine in modern history that's been as significant to motorsport (but maybe I'm biased). It's the ultimate Meccano set !

Regards, Paul

[CBUEB1771]

I flipped through the Dave Bean catalog and in the connecting rod section saw that the 4.826"rods are attributed to Cosworth and used in TC and MAE engines. I wonder if the MAE was the motivation for this connecting rod. I may have details wrong but the MAE was the one-liter F3 engine built from 109E blocks. The 109E had 1.3 liter displacement so a short stroke crank shaft and long connecting rod would have been needed to get the correct displacement and getting the piston where it is needed at the top of the stroke. The 109E connecting rod length is 4.284" according to the Bean reference.

[rgh0]

The forged JE pistons I have used in a number of engines for both road and racing over the years are certainly suitable for road use with the right intruder height for the right road fuel compression ratio.

No issues with wear or piston slap or oil use if built with the right clearances and bore machining .

cheers
Rohan