Ever since I got my engine back from rebuild a year ago it's had a bit of a tappetty sort of rattle. At Techenders I was reassured it's not a problem as such. I've adjusted the valves umpteen times at 0.15 mm using the conventional TDC on cylinder 1 with both valves closed then working round 2, 3 and 4. I also used a different method described in Tom Wilson's book. Yesterday I took the left hand valve cover off after the engine had warmed up and checked the clearances. They were all around 0.2 mm, hence wider than when cold. I thought they were supposed to get tighter when hot, so what's happening?
The difference in the expansion of an aluminium pushrod and the cylinder should have the valves closing up by about 0.1mm per 100 degrees C .. Based on quick calculations assuming they are approx 20cm long, the cast iron cylinder is 15mm high and there is 5cm of aluminium or magnesium in the engine block below the barrel and the cylinder head.. I think my dimensions are a little small and the general change in the valve clearance maybe a little more. But if your pushrods are steel, (stiffer, maybe sound different) then the pushrod expands less than the aluminium in the stack under the piston..approx 0.05 mm less if the aluminium/magnesium expands twice as fast as cast iron/steel.. Which is why if you have chromoly alloy steel pushrods, the advice is to set smaller clearances as they widen up as the engine block and cylinder head expands...maybe thats it..
Thanks Mike that would certainly explain some of it. I had thought my push rods are aluminium though (late type 4 engine); I need to check.
How late is the late Type 4? If it’s a ’79 it could have a hydraulic cam and followers – steel pushrods and spacers instead of springs on the rocker shafts.
No it’s not that late. It’s got non-hydraulic solid lifters. I’ll have to check whether the push rods got changed in the rebuild.
The rest of the engine is expanding more than your pushrods .. Why would you measure them when warm ?
Reading the OP its getting clattery when hot, opposite to expectation. So measuring hot makes sense. Now is the time to try a pushrod.. see if the centre section is magnetic - if it is then its some kind of steel . So it expands at half the rate expected for aluminium. So the less clattery solution for steel pushrods with solid lifters is to e.g. set to a "loose zero" and check when hot..
Could also be cromoly pushrods. I have heard of issues with them not being set right and causing problems
Yes cromoly are a steel alloy so expand about 50% the rate of aluminium. So there are no official settings, but the lore is the "loose zero" when cold as the gaps can only get bigger on average (depends on the sequence of components warming up before reaching steady state temperatures) . This is all because of the expansion rate of the cylinder head and the engine block is about twice as fast as the steel or cast iron. This is why hydraulic adjusters in good condition with low enough viscosity oil are a good idea, they pump up and leak down gently to track changes in the dimensions of the valve train.
So thanks for the suggestions about push rods. These have indeed been changed from the original aluminium ones. I've adjusted the clearances to 0.1 mm to see if it makes any difference.
If they’re Chromoly that’ll be why the clearance opens up when hot. 0.01 is too loose and they’ll still rattle, get the cold clearance as near zero as you can – a loose zero. What else has been changed to the engine spec? Chromoly pushrods usually means a cam with aggressive ramps and stronger valve springs.
or builders that don’t know or care and bang them in as they are “better” than stock. and this is where problems begin I mentioned it because when my car was getting its motor a distraught owner came in as her “new” motor was making all sorts of noises. Couldn’t take it back easily to builder as he was 4,000 km away. mate didn’t want to touch it as he knows the builder. one of the issues were the cromoly push rods that just didn’t need to be there
I think based on the metals, you can set them to zero and then they will still be more clattery when hot than the aluminium ones.
I'd be getting some ali push rods. Strange thing for the builder to do. Either ignorance or slap-dash? Neither bodes well for the quality of the rebuild. Perhaps I'm missing something.
Hydro pushrods (steel ) are 9mm shorter than the aluminium pushrods used with solid lifters . I’d pull a lifter and rod and check lengths and make sure they are solid lifters . Valve clearance is set on stone cold motor and some of the above advice is type 1 engine related.
Thanks for the comments. I've now got a push rod out. Although I said above that they've been changed, I find that they are in fact aluminium and not new so probably my originals. The dimensions are 12 mm diameter 270 mm long. I don't think I'll be needing yours @DubCat but thanks for the offer. I've had a replacement stock cam, new B&P's and new heads so can see the potential for different expansion rates. With the valves set to 0.1 mm cold it sounds better. I might try a bit less but it runs fine.
Also when I got my engine back from rebuild it had valve covers fitted, although I sent it in without them. The covers have a VW logo on so I'm assuming they are not some fake replicas, but they are lighter gauge metal than original (334 vs 428 grams). I've fitted the originals and this actually deadens the tappety noise a bit.
