Lotus Elan

Short jackshaft install

PostPost by: l10tus » Thu Apr 23, 2020 1:00 pm

Thinking about installing a shortened jackshaft in my refurbished T.C. engine.

Has anybody used one of these and is perhaps willing to share their installation / conversion experience?

Especially in describing the necessary work in order to blank out the oilways in the rear bearing, etc.

Replies & thoughts appreciated.
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PostPost by: Pfreen » Thu Apr 23, 2020 1:21 pm

I have one in my tc. I just cut off the fuel pump end of the shaft.

To plug the oil journal hole off the unused journal, install a bearing shell but be sure to not align the bearing oil hole with the journal oil hole. This blocks the oil.

You must also blank off the fuel pump surface on the block.
Last edited by Pfreen on Fri Apr 24, 2020 8:15 pm, edited 2 times in total.
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PostPost by: 2cams70 » Thu Apr 23, 2020 1:48 pm

I did the same to my engine - a couple of thoughts.

1. You are better off shortening an original jackshaft rather than purchasing new.
2. Best to replace all three bearings as a set and turn the last bearing so the oil gallery here is blanked off. You are better off having the bearings installed professionally rather than trying to DIY. The alignment tolerances to get a free running shaft are very fine and the bearings will likely require a light line bore after installation. Take the shaft you are going to use in with you at the same time to the machine shop so they can check the fit. Just knocking the old ones out and knocking the new ones in yourself is not likely to result in a smooth free running shaft. If you can't be bothered doing that and the existing bearings are in reasonable condition leave the first 2 alone and just replace the last bearing and turn it so the gallery in the block is blanked off.
3. Recommend to use a steel thrust plate from Burtons, QED etc. instead of the standard sintered one. They are ground to a tighter tolerance and will reduce the shaft end float to the minimum range - i.e 0.002" instead of the 0.006" that is typical of the standard thrust plate even when new. Less end float here = less ignition timing scatter.
4. If it's a street engine any performance improvement probably won't be noticeable and you won't be able to return to a mechanical fuel pump of course.

Note the reason why I say to use a shortened original shaft rather than a new one is that my experience with new ones hasn't been good. The first one I tried had not been properly linished after the oil slots had been cut (not easily detectable by eye) which resulted in my nice new bearings being scored. The second replacement one had a slight bend at the front which resulted in the end float varying between 0.000" to 0.002" depending on where it was measured with the feeler guage. I gave up in the end and used a shortened original.
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PostPost by: benymazz » Thu Apr 23, 2020 3:58 pm

I totally agree with what 2cams70 said. What I did when I did this to my engine last year basically mirrors what he said.

I took my original jackshaft, cut it just after the second journal with a hacksaw, then took it up to my lathe and faced off the end. Had my crankshaft going to the machine shop that week so I threw the shortened jackshaft in as well and had my machinist grind off the lobes (he was skeptical about turning them off on a lathe; said that since they were hardened they would need to be ground off).

While my block was being bored I had them replace the jackshaft bearings. They didn’t change the last bearing, only rotated it in its bore to blank off the oilway. Like 2cams70 said my machinist told me that it might require a light line hone to get the right clearances with new bearings but I got lucky and didn’t need that. When replacing I used a new steel endplate.

Of course this is assuming you do all your homework and the journals, retaining plate groove, and helical gear on the existing jackshaft are within spec. Otherwise you just have a nice cast iron paperweight.

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PostPost by: rgh0 » Fri Apr 24, 2020 10:00 am

I have purchased new steel short jack shafts - you will see one soon in my next post of my race engine strip and also cut and shorten original iron shafts - both work ok assuming you have a shaft that is dimensionally correct to begin with . The short steel shafts come with no cam lobes with "adds lightness" or you can machine the lobes off the remaining length of a standard shortened shaft but this will be challenging as they are chilled cast iron and very very hard and really need to be ground off

Installing the jack shaft bearings takes care but can be done by anyone with a suitable sized socket and extension shaft to drive or press the middle bearing into position and the first and third ( suitably rotated to seal the oil gallery) driven or pressed with a wooden block. The edge of the bearing can be burred in this process and generally just cleaning the edge results in a clean running jack shaft, No need for line boring of the bearings

Note there are various different arrangements for the jack shaft flats that feed oil to the head. The push rod engines had only one slot machined Lotus twin cams had 3 to feed more oil to the two cams. The BDAs as I understand it had a single slot but a groove to feed the additional oil to the head. Most of the steel jack shafts I have seen have the BDA arrangement.

cheers
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PostPost by: Craven » Fri Apr 24, 2020 11:11 am

After a couple of failed attempts to fit them I turned up these guides, one to locate the other to drive in the bearing worked perfectly.
Jack bearing.JPG and
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PostPost by: 2cams70 » Fri Apr 24, 2020 11:14 am

Clive Cams here in Melbourne ground the lobes off my jackshaft for an entirely reasonable $60 (not that it's really necessary - more because I'm a bit anal retentive!) Why would you bother attempting to turn it down in a lathe yourself for that small change??

Same with the jackshaft bearings. Had them installed to perfection by Crankshaft Rebuilders for $100. That's pretty cheap in the context of an engine rebuild. My experience is that they aren't particularly easy to install correctly and to get a free running shaft. They certainly require a bit more force than that provided by a block of wood to drive in and you certainly need a correctly sized shouldered mandrel to drive them in straight and without damage rather than just a big socket. Another set of bearings is $60 if you damage them during installation. Just not worth the risk in my opinion.
Last edited by 2cams70 on Fri Apr 24, 2020 11:56 am, edited 1 time in total.
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PostPost by: 2cams70 » Fri Apr 24, 2020 11:55 am

Craven wrote:After a couple of failed attempts to fit them I turned up these guides, one to locate the other to drive in the bearing worked perfectly.
Jack bearing.JPG


I like your idea but I'm time poor and don't have access to a lathe!
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PostPost by: Craven » Fri Apr 24, 2020 12:50 pm

2cams70 wrote:
Craven wrote:After a couple of failed attempts to fit them I turned up these guides, one to locate the other to drive in the bearing worked perfectly.
Jack bearing.JPG


I like your idea but I'm time poor and don't have access to a lathe!

OK, if you have not done this job yourself then you may not realise these are split bearing and very easily twist out shape if not kept true. Bearings are available in size and those that require reaming after fitting.
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