1963 T6 B Super Pushrod/Valve Clearance Issue
Posted: Tue May 22, 2018 4:37 am
Hello fellow 356ers,
I am currently rebuilding the top end of my 1963 T6 B Super engine at a friends workshop and have run into the following problem.
The last rebuild of the matching numbers engine was well over 120,000 miles ago. It was a runner before our disassembly. We're only redoing the top end - bottom end has been left intact.
Crank end play is within spec, cam still looks good, followers look fine, connecting rods look fine, small end bushes are tight and the new big bore kit fitted beautifully.
Heads have been flycut previously
Heads have now been completely refinished by an outside machine shop due to having 3 cracked combustion chambers
All new valve guides, intake and exhaust valve seats and intake valves. Exhaust valves were ok, as they were replaced a few years ago. All existing valve springs were within spec and reused.
Fitted new big bore forged pistons and AA 86mm biral cylinders - replacing the previous cast iron 86mm big bore kit.
Deck height appears to be matched to the previous deck height and we have shimmed the cylinders accordingly - although we have not verified deck height at this point, but we will do so. Let's assume the deck height is ok for the sake of this discussion - the crank turns over freely.
My guess is that as a result of adding new material to the heads to repair the cracks and cutting the new valve seats, the valves now sit higher in the combustion chambers than they used to - and with wear, they will continue to move upwards.
With the newly rebuilt heads, there is now insufficient adjustment available on all of the rocker arms (with the old B super pushrods fitted) to attain correct valve clearances, ie the rocker adjustment has been backed off to the max and correct valve clearances are unachievable on both the intake and exhaust sides of both heads.
The existing (the old) pushrods measure between 256mm and 257mm.
I am looking at fitting new (shorter) pushrods to this engine so I can achieve the correct valve clearances. I would rather do this than shim the aluminium rocker stands.
The questions are as follows:
1. Just how much adjustment is reasonably available in the rocker arm assemblies on both the intake and exhaust side? I'm aware that with extreme rocker geometry here there is a potential oil starvation issue, so I want to reset the valve adjustment geometry to sit somewhere in its midrange.
2. Is there any unusual issue that you need to take into account when you reduce the length of all pushrods in a 356 engine?
3. I've searched the web and can find 2 shorter pushrod lengths for 356 engines. One is 255.7mm, the other 252.5mm. 255.7 seems not enough of a variation, whilst 252.5 seems too much of a variation.
As we are still in the test fitting phase of rebuilding the engine, any words of wisdom on this subject would be greatly appreciated.
I don't want to mess this up.
Many thanks
Richard
I am currently rebuilding the top end of my 1963 T6 B Super engine at a friends workshop and have run into the following problem.
The last rebuild of the matching numbers engine was well over 120,000 miles ago. It was a runner before our disassembly. We're only redoing the top end - bottom end has been left intact.
Crank end play is within spec, cam still looks good, followers look fine, connecting rods look fine, small end bushes are tight and the new big bore kit fitted beautifully.
Heads have been flycut previously
Heads have now been completely refinished by an outside machine shop due to having 3 cracked combustion chambers
All new valve guides, intake and exhaust valve seats and intake valves. Exhaust valves were ok, as they were replaced a few years ago. All existing valve springs were within spec and reused.
Fitted new big bore forged pistons and AA 86mm biral cylinders - replacing the previous cast iron 86mm big bore kit.
Deck height appears to be matched to the previous deck height and we have shimmed the cylinders accordingly - although we have not verified deck height at this point, but we will do so. Let's assume the deck height is ok for the sake of this discussion - the crank turns over freely.
My guess is that as a result of adding new material to the heads to repair the cracks and cutting the new valve seats, the valves now sit higher in the combustion chambers than they used to - and with wear, they will continue to move upwards.
With the newly rebuilt heads, there is now insufficient adjustment available on all of the rocker arms (with the old B super pushrods fitted) to attain correct valve clearances, ie the rocker adjustment has been backed off to the max and correct valve clearances are unachievable on both the intake and exhaust sides of both heads.
The existing (the old) pushrods measure between 256mm and 257mm.
I am looking at fitting new (shorter) pushrods to this engine so I can achieve the correct valve clearances. I would rather do this than shim the aluminium rocker stands.
The questions are as follows:
1. Just how much adjustment is reasonably available in the rocker arm assemblies on both the intake and exhaust side? I'm aware that with extreme rocker geometry here there is a potential oil starvation issue, so I want to reset the valve adjustment geometry to sit somewhere in its midrange.
2. Is there any unusual issue that you need to take into account when you reduce the length of all pushrods in a 356 engine?
3. I've searched the web and can find 2 shorter pushrod lengths for 356 engines. One is 255.7mm, the other 252.5mm. 255.7 seems not enough of a variation, whilst 252.5 seems too much of a variation.
As we are still in the test fitting phase of rebuilding the engine, any words of wisdom on this subject would be greatly appreciated.
I don't want to mess this up.
Many thanks
Richard