Twin cam Valve Guides.

PostPost by: SJ Lambert » Fri Jul 20, 2012 6:36 am

Thanks for that Rohan, I presume that Orger Engines or even Crankshaft Rebuilders in Melbourne could handle the Colisbro guides if we go that way?

I wonder whether the reaming could be done in the garage if the correct tungsten reamer could be obtained? Ayross might be able to supply reamers............

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PostPost by: rgh0 » Fri Jul 20, 2012 7:15 am

Orgers can certainly do it, they have done for me many times.

While you can do if you had right reamers, their Serdi machines which do the seats and guides properly to get parrallel and concentric are hard to duplicate in terms of the accuracy with a home garage job


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PostPost by: SJ Lambert » Mon Jul 01, 2013 4:53 am

G'day Rohan

Do any of the shops have a machine that can slim old valve stems down to 7mm?

Am thinking of running slim stemmed valves - may even have to source colisbro bar stock in order to be able to get the right ID..........

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PostPost by: rgh0 » Mon Jul 01, 2013 9:13 am

Hi James

7mm valve stems are a relatively common mod. Talk to Orgers as they may be able to supply what you need. Some of the other race engine parts suppliers overseas may also do them if you ask. I have heard of people using Gemini valves ( If I recall correctly) which have 7 mm stems. You could cylindrical grind a standard stem but you would need to know the details of the surface hardening and whether this would be affected and have to be redone.

In general I don't believe the smaller stems add much and you give away some life of the valve stem and guides. The guides are short and making the stem smaller in diameter decreases the bearing area and will accelerate wear. However its hard to tell if it really matters as most race heads get rebuilt long before guide and valve wear is a problem. Most good inlet valves ( which is the only place it matters) have a reduced stem in the port area in any case and you are only getting the benefit of the small length of larger diameter guide running stem that is exposed on full lift. The weight saving is not significant compared to the rest of the valve train.

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PostPost by: tract0rman49 » Mon Jul 01, 2013 10:03 am

Hi There

Regarding the "Reamed" Valve Guide size and tolerance.
I am an apprentice trained mechanical engineer and recall that during my apprenticeship and metrology classes at Technical College (which spanned the late 1960's) measurement was not quite the advanced science it has since become with the availability of CMM (Coordinate Measuring Machines) measuring to around 3 Microns accuracy.
One Tenth of one thousandths of an inch is approximately 2.5 Microns and the idea that Lotus could realistically measure a nominally cylindrical bore of a valve guide to a tenth of a thou (Inches) is ridiculous. Nominally a 5/16" reamer (0.3125") is what you need and in my opinion this needs to be done after fitting the guides to the head. All lotus twin cams burn oil and getting oil down the guides does a lot more than just lubricate...primarily oil in the engine is the first means of transferring heat away from the source of the heat (exhaust gases in this case).
50 years of progress and improvements in machining equipment and metrology are the reason why today's engines don't burn and leak oil in the way that cars did when Elans where designed an built.
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PostPost by: SJ Lambert » Tue Jul 02, 2013 10:21 am

Drat, got a set of Kent titanium retainers for a pushrod motor to suit 9/32" stems. Presumed, incorrectly, that matching collets would accompany them.......

I've asked Burtons which ones will match the retainers, looks like there's a single groove YB one that may match.........

Edit- I've also contacted Kent Cams directly re collets to suit their VRT 05 retainers.
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PostPost by: rgh0 » Tue Jul 02, 2013 2:06 pm

You need to match the retainers to the springs you are using. There are 2 diameters of spring people typically use and the step to the inner spring varies a little also. Plus some retainers suit top hat shims and some suit flat disk shims. Plus some retainers are better than other in terms of relationship of spring top versus valve top. Lots of possible choices if you want to mix and match but most people will sell you a spring pack of springs and retainers and you can get collets to suit the stem diameter you are using as most retainers use the same collet design.

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PostPost by: SJ Lambert » Thu Jul 04, 2013 5:59 am

I've matched springs to both retainers and camshaft (all Kent items). Kent have advised Pinto collets (I think Pinto stems are 8 mm) & Burtons have advised cross flow collets (ie to suit 5/16" stems).

My research on Burtons site reveals 2 much more likely candidates for 9/32" stems - BDA Collets suit 9/32 stems and as mentioned earlier some Cosworth YB valves are on 9/32" stems. Both BDA & YB collets are single groove items so it's less work if I end up turning valves from blanks.

There's also a YB collet that's available to suit 7mm stems - so they may be the one's that have gone into my Twin Cam head......
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PostPost by: rgh0 » Thu Jul 04, 2013 9:12 am

Out of interest what are the specifications for the springs and Kent cam you are using?


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PostPost by: SJ Lambert » Thu Jul 04, 2013 11:34 pm

Springs are VS39, installed height 34.0mm, installed pressure 90lbs, min height 21.00mm, max pressure 250 lbs, coil bind @20.0mm, max lift 13.00mm.

Cam shaft is part no. 272 overall duration in/ex 314 deg 54/80 90/44 cam lift in/ex 8.81mm for a nominal valve lift of 12.82mm.

I've modified the 120E block to take the thicker stemmed cross flow followers as they're considered by some to be able to take high revs more reliably.
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PostPost by: rgh0 » Fri Jul 05, 2013 8:10 am

The post below I made in error - I thought James was talking about a twin cam head not a push rod head
sorry for the confusion

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James - I normally would not use this sort of setup for a variety of reasons. I will find some time to post them this weekend I hope and we can discuss if they are relevant to your particular build. Its always hard to comment without looking at and measuring up the overall engine and head in the flesh and then doing a whole bunch of analysis and calculations particularly when you are building an engine meant to run to the limit of whats possible on the track

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PostPost by: rgh0 » Fri Jul 05, 2013 11:11 am

I have decide to start a new thread about "high lift twin cam valve train design" to address some of the issues touch upon in this thread and others I have posted to over the years.

So find that thread to see how this discussion continues

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PostPost by: SJ Lambert » Fri Jul 05, 2013 10:04 pm

rgh0 wrote:James - I normally would not use this sort of setup for a variety of reasons. I will find some time to post them this weekend I hope and we can discuss if they are relevant to your particular build. Its always hard to comment without looking at and measuring up the overall engine and head in the flesh and then doing a whole bunch of analysis and calculations particularly when you are building an engine meant to run to the limit of whats possible on the track

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Rohan



I'm all ears on correct cam selection Rohan, this pushrod motor is destined to go into the Elfin 300, it's on a 53mm stroke, very long rods & have yet to start building a downdraft head for it. I will use period Holbay Rocker gear and have pushrods that I can optimise their length to suit the rest of the package.

Carburation is 2 by 40 mm IDFs.


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PostPost by: rgh0 » Sat Jul 06, 2013 2:10 am

Hi James

I need to retract my comments - I thought you were talking about your twin cam that your building on the McCoy head not your push rod motor for the Elfin. My comments were around a twin cam head. For a push rod head I know a lot less and what you have selected looks OK in my limited and mainly theoretical experience with pushrod motors.

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PostPost by: SJ Lambert » Sat Jul 06, 2013 3:29 am

The McCoy T/C head's got John's 510 & 480 cams, his springs & valves at 1.65" & 1.45" on 7mm stems.

Re the pushrod motor, there's not been a whole lot of science from me in selecting the 272 race cam. I'd heard a few people poo poo the Isky springs for push rod motors, the head honcho at my machine shop reckons Kent do the best cams for pushrod Ford motors so I chose one with plenty of lift, they do offer a couple of longer duration, like the 658 which is a 320 degree overall duration. I think that it & the 747 grind have been around a fair while. There's an A8 & an A8+ still available these days & I presume they're based on Cosworth grinds. The A8s are 312 overall but with less lift. The 272 is listed as "working" to 9300 rpm which is a better top end than any of the others & as far as I can tell, it's one of their latest offerings. I figured that their recomended springs would be the way to go with their cams.

Besides putting a head together that flows fabulously in order that the best can be brought out of the cam I reckon we'll have to be meticulous with valve train geometry in order to avoid problems whilst accommodating the high lift ......
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