| Author |
 Topic  |
|
|
graham4233
United Kingdom
47 Posts |
 Posted - 09/04/2008 : 09:36:47
|
I am about to assemble my NA head for the first time. New guides, valve and seats lightly cut and two new exhaust valves. Valves and seats lapped together. By reading through the forum I understand the importance of No 1 at TDC with dynamo yoke fore/aft and the corresponding cam lobes as a Curchillian V. I know that the position of the eccentric bronze being bush parallel to the head is important for timing and that valve clearance is initially achieved by grinding the valve stems. What I don't know is how to do it!
What is the assembly order? Valves first, cam stands first? Do you take the initial valve clearance measurement with or without springs fitted. What is the valve clearance cold on the bench to achieve the 6 and 8 thou hot? What tool to use to get an even torque on the head nuts and so on!?
Some advice needed or where I might obtain the information. Cheers Graham. |
|
|
talbot
United Kingdom
718 Posts |
Posted - 09/04/2008 : 12:17:39
|
Graham,
I've recently completed a rebuild of my cylinder head. The ideal set up is to have the bronze rocker bushes as per the diagramm with a 6 thou clearance. You do this by either grinding the valve / seat - to reduce the gap, or by grinding the end of the valve stem to increase the gap. To make life easier I built up the cam and rockers etc. with the bronze bushes correctly positioned but without the valves. I then inserted one valve and held this tightly closed with a small plate, across the combustion chamber, secured with two bolts through head stud holes. I was then able to measure the clearance and very simply remove the valve for the appropriate grinding. This method means you don't have to keep dismantling the cam and springs etc. After the final assembly I needed to just give the bronze bushes a wisker of a turn to give me 6 thou clearancee. I set the timing as per Peter Green's advice on the Churchillian V but after assembling the dynamo and setting the crank to TDC one cam was slightly lower than the other. I simply removed the top half of the cam bearings and rotated the cam by one tooth. One other thing you should do is check to make certain the oil holes in the bronze bushes line up with those in the rocker shafts. I managed to find a set of original spacers that fit between the bushes and when I trial assembled these without the rockers I found a couple of the holes didn't line up.
Good luck
Jan T
|
 |
|
|
ags
United Kingdom
275 Posts |
Posted - 09/04/2008 : 13:46:47
|
Hi both,
Be very careful when turning the cam! As I pointed out in a post a little while ago, referring to cam gear timing marks, there is a vernier type of adjustment between the pinion/cam gears and the crank/dynamo gears. In other words in your situation you may have got the dynamo meshed wrongly.
Look for all the timing marks and set those together should be the first step in my opinion. The "rabbits' ear" or "Churchillian V method" should only be applied if you are confident that the rest of the settings are correct. I know, having tried it, that with the settings out by one or two teeth there is little difference in the driving feel of a four. Take a stopwatch with you and the differences are seconds, say ten percent, to carry out a given acceleration. If you are happy with that, fine, but please do not give our cars a worse reputation than they already have.
As to the length of the valves, the Works service sheet 7J, issued April 1933, gives the correct figure for Js and a method of setting all the valves on the bench without needing constant assembly plus dismantling. The valve length needs to be adapted for the greater thickness of the large cam heads (see Blower for these details) and I adapted the setting gauge to be a simple tube, because with my skill limits and equipment, I was more confident of filing two ends of a tube accurately than the bridge type gauge which needs true and correctly positioned faces in four places rather than two which can be easily checked by vernier caliper or micrometer. I strongly suspect that Mike Hawke will have more relating to this subject in his recent book on the J2, see this month's Safety Fast. Anyone who needs a scanned copy of the service sheet should send me an email and I will send one back by attachment to my direct reply.
Our cars were designed for skilled manual assembly and this is still needed.
As regards torquing the head nuts Gordon's tools used to sell a form of square drive spanner they called a crow's foot. This was in effect an open ended spanner head offset a short distance from a suitable square drive aperture. With this, and a normal short extension plus a square to square offset you can, and I do, torque to your heart's content.
This will have to count as a technical rant from,
Andrew Smith MMM571 |
|
|
|
Gerhard Maier
Germany
874 Posts |
Posted - 09/04/2008 : 14:08:57
|
Graham,
To find the timing-position for the camshaft, I know two methods:
To get the position accurate to about 1.7 degrees, you have to play with the gear-positions of the dynamo and of the vertical drive, until inlet opens at 15degrees B.T.D.C., which is 1 5/16 on flywheel.
(Send me a mail, if you want a sketch for that)
But to be honest this method only has a advantage if you want the last percent of the possible power.
The method which really many of us use, is to adjust the position of the cam lobes of cylinder one (at firing stroke). You put them into the 10 and 14 o'clock positon (you call it "Curchillian V"), when the piston is on TDC.
With that method you need not to care at all about dynamo-fork positions whilst fixing the dynamo. Forget all arrows on gears, connect dynamo and vertival drive with the coupling steel-disc.
And only now, when you are fixing the camshaft, put the lobes in the position mentioned above.
When you place a steel-ruler over the lobes, the distance from the ruler down to the cyl.-head-cover gasket face should be about the same on both sides.
Often you can't achieve this exactly, because one tooth of the gear is a lot. In that case chose the better timing position, which is when the ruler is hanging down slightly on the left side (in driving direction).
Concerning the positon of eccentric bushes: After adjusting the valve clearance by grinding the valve stem as "talbot"describes, I give the flat of the bushes a slight angle to the "wrong"side, so that during the first adjustment, when everything has settled after a few miles, the flat comes into the ideal level position.
The tool I use for tighting the head nuts with a torque-wrench, is a cranked HAZET-tool Order-No. 329-2, which needs grinding from 14 to 15,4mm (Send me a mail, if you want a photo of that)
A word to the fixing of the most forward camshaft-stand:
When you use two screws instead of the original two bolts with nut,
you easier can remove this camshaft-stand by pulling it forward, for example in case of a change of rocker-shafts.
Gerhard
|
|
|
|
ags
United Kingdom
275 Posts |
Posted - 09/04/2008 : 16:18:19
|
Hi again all,
Gerhard and I clearly disagree about the magnitude of the effect of timing errors on power output, he says "last percent" and I say "about ten percent". I do agree with him that a straight edge over the cams should not be absolutely horizontal in the best position. This follows from the fact that the cams on our machinery are shaped symmetrically, but the timing of the two valves is not equally spaced either side of TDC, as he says the inlet should open 15 degrees BTDC but the exhaust should close 20 degrees ATDC.
I do still maintain that the easiest way with an engine on the bench is to clean all the gears, find the marks and clarify them with white paint and set them together as Blower describes. Anything else is guess work, and the factory, even with all their experience, guessed wrong quite often. Hence the "quality control" problem mentioned by Mike Hawke and one element of the poor reputation of J2s in particular and all our cars in general.
I for one will do anything I can to encourage people away from producing "gutless buzz boxes" to quote on phrase which I have heard from VSCC pomposities.
Ranting over and out,
|
|
|
|
tholden
United Kingdom
1638 Posts |
Posted - 09/04/2008 : 23:48:28
|
Andrew I think you can only use your timing method of lining up the original marks if you are absolutely sure that all the components are original and also original to your engine and also correctly set up by the works in the first place. I dont see that it matters how the vertical drive is connected or how the teeth are meshed as long as the relationship of the camshaft timing to TDC is correct.
The other factor involved in all this is the accuracy of your camshaft, rockers and rocker bushes. If you are using replacement parts and they are not manufactured to precisely the original spec you will probably not achieve accurate valve timing even if you do set up your head correctly as per the book.
It must also be remembered if using the inlet valve opening point to set your valve timing that new camshafts are now available with different valve timing to the original.You will need the camshaft manufacturers design figures if you are to set your camshaft timing correctly using this method.
Modern camshaft manufacturers supply a camshaft timing figure which represents the number of degrees that the camshaft produces full lift after tdc. To calculate this you need to know the actual duration and precise inlet opening degrees of the particular cam you are using.
Given the likely missmatch of old and new components in most rebuilt engines and the variation in tolerances in the replacement components
I suspect that the most accurate method of setting your valve timing is as Gerhard states to use the V method probably with the straight edge slightly down on the left side. The advantage of this method is that it compensates for all the likely variations in the components and provides the best compromise timing.
Having set up your head using this method there remains the potential to play with your timing a few degrees one way or the other by altering the gear meshing top and bottom to achieve maximum power from your engine. I have certainly achieved improved power from a supercharged engine in this way.
Terry Holden |
|
|
|
kimber
United Kingdom
1529 Posts |
Posted - 10/04/2008 : 06:57:54
|
I sort of agree with both but I think ags is right if we are talking about all original equipment.
The point is that if the bottom bevel gear set is one tooth 'out', no amount of playing with the meshing of the top bevel gears will enable you to get the valve timing in the ideal/correct/optimum position.
I am sure a lot of people put a lot of effort(as I have done and still do) into trying to get this right whenever they re-time their engine for whatever reason and it is very easy to get wrong, or nearly right.
I don't think the importance of getting it absolutely right should be understated. With the valve timing spot-on these engines really sing. Even with it slightly out, they will go almost as well. My own road-going P-type is running now with the timing incorrectly set simply because I can't find the time to pull the dynamo out. The car is still perfectly OK to drive but I can tell from the sound and feel of it that the timing is not spot on. Without looking, I'm willing to bet that the problem is one tooth wrong at the bottom compensated by one tooth adjustment at the top which equals 'nearly right'.
I think that the difference is probably more noticeable on an un- blown engine. |
|
|
|
graham4233
United Kingdom
47 Posts |
Posted - 11/04/2008 : 19:57:53
|
Gentlemen, armed with your helpful information, and that of other posts, I assembled the NA head. With my limited experience and working with the rockers, bushes and shafts that I had I managed a reasonable compromise with the valve timing. No more than half a flat of the bushes to finally get the 6 and 8 thou. The dynamo bevel was one tooth out, now rectified, and with the Churchillian V set and the VD drive yoke across the car the flexible coupling married up. The car pulls better and is crisper from 3000 rpm so I am pleased with a little progress. As I am still getting a bit of exhaust smoke looks like a rebore and new pistons next winter. When will the kids leave home and finish uni!
Cheers Graham |
|
|
| |
Topic |
|
|
|
Forum Locked
Topic Locked
