Lifter preload

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gd302
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Lifter preload

Post by gd302 » Fri Mar 09, 2018 8:24 pm

I have been checking my lifter preload over the last few days on my 3.9 Tvr engine refresh.
I used the method of tightening down the rocker shafts equally until all play disappears, and then measure the gap under the pedestal. This gap multiplied by the rocker ratio should give me the preload on the lifter.
I could not get any accuracy using wire to measure at the lifter itself.
I obtained Similar measurements of around 105 thou at the pedestals, multiply this by 1.6 and I have 168 thou at the lifters.
I understand it should be closer to 60 thou at the lifter, so I installed some 60 thou shims under the pedestals and now have about 70 thou at the lifter.
In reality am I ok to leave it at this or should I aim to get it down closer to 40 thou?
I’d be a bit concerned over the geometry if I go too far.
It would have been interesting to see what they were before I stripped the engine but we never think of these things.
I have kept the block standard and added some stage 3 heads and an MC1 cam. Standard pushrods and lifters.


1995 TVR Chimaera 400 With tweaks

sidecar
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Re: Lifter preload

Post by sidecar » Sun Mar 11, 2018 4:55 pm

The engine will work well enough with preload figures that are all over the place, the standard valve train geometry is not very good anyway, adding shims makes it worse. If you are going to the trouble of sorting this type of stuff out then the pedestals need shortening and adjustable pushrods are needed. The less preload you use the less the lifters can pump up and hold the valves off the seats (This was the crude rev limiter that Rover used). I use about 25 thou preload. If you are running a long duration cam then this sort of stuff can become important because if the lifters have pumped up the valves could be open at least 2mm further than they normally would be just around the time that the valve to piston clearance is getting tight, 2mm could mean that the pistons hit the valves. I have built many engines where this sort of stuff has become important and it all needed measuring. (which takes ages). I have also built fairly standard engines where none of this stuff mattered because the standard setup will always have enough tolerance to allow you to 'throw' the engine together knowing that it will run without any issues. As you are using a non standard cam you should consider looking at the valve to piston clearance when the valves are 'on the rock' Unfortunately in order to do this you need two lifters that have been made solid and two adjustable pushrods. I don't know the spec of your cam but in my experience it does not need to be a very radical cam before the valve to piston clearance gets too tight in my humble, you then need to pocket the pistons in order to create some clearance. (Pocketing the pistons is a pain in the arse but then again so are bent valves!). You could ignore all this extra work and you might be OK, its up to you!

gd302
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Re: Lifter preload

Post by gd302 » Mon Mar 12, 2018 6:18 am

Ok thanks for that.
1995 TVR Chimaera 400 With tweaks

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DEVONMAN
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Re: Lifter preload

Post by DEVONMAN » Mon Mar 12, 2018 9:57 am

It sounds like your stage 3 heads have had a sizeable skim and also the valve seats have been recut deeper into the head. I would be considering adjustable push rods because as sidecar has said, shims will make the RV8 rocker geometry even worse and a large preload setting will allow greater pump up (at high revs) of the lifter and increase the risk of piston contact. Are you using composite head gaskets?
1950 A40 Devon Hotrod with 5.0 twin turbo RV8. (10 things to make you smile)
EDIS8 wasted spark, Holley Injection.
Been as far as the Moon and back in 54 year of driving. Same Car, 5 engine upgrades !!!


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gd302
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Re: Lifter preload

Post by gd302 » Mon Mar 12, 2018 12:57 pm

Yes composites. Its not an engine I intend to abuse revs wise but the article on V8D website seems to suggest up to 100 thou is acceptable on the preload.
1995 TVR Chimaera 400 With tweaks

sidecar
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Re: Lifter preload

Post by sidecar » Mon Mar 12, 2018 3:11 pm

Like I said any old preload will work BUT you run a risk if you have a large amount of preload, the risk is that if the valves float at anytime then the lifters will pump up, once they pump they will hold the valves off the seats in order to cause the engine to misfire, this isn't the real issue, the real issue is that lets say at 20 degrees BTDC on the exhaust stroke the valve would be open lets say 4mm, now if the lifter has pumped up the valve could be open 6mm and that might be enough for it to hit the piston. Adjustables are quite expensive but they are SO easy to setup I always use them an any half decent engine that I'm building. In order to set them up you just adjust them to take out all the slack and then just adjust them a 'number of flats' of the adjuster nut. From memory one flat is worth about 5 thou of preload, I set mine up with five flats. (5/8 of a full turn of the nut). Usually the engines that I build need the push rod holes opening up and the actual pushrods usually need to be shortened about 5mm before the ball end is fitted to the pushrod.

In my first post I forgot to mention that you also need weak checker valve springs in order to carry out the valve to piston clearance checks, old rocker shaft spacers springs cut in half make good weak checker springs!

gd302
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Re: Lifter preload

Post by gd302 » Tue Mar 13, 2018 7:46 am

Thanks both, great advice.
1995 TVR Chimaera 400 With tweaks

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