MGB & GT Forum

The other day there was a discussion about Smiths Brothers pushrods which leads me to my question.

BWOB before I came to MGE Peter Burgess' book was my go to. In it he notes that if the head has been skimmed you may need to adjust the length of the pushrod to achieve the original clearances - I'm paraphrasing him. He does open his words with "blueprinting the valve train" so if I interpret that correctly this is only necessary when you are actually blue printing which is not the everyday way of going about this - it's specialised rebuilding. And perhaps that's why both manuals I read do not cover this aspect. My head has been skimmed (by Peter) but not heavily skimmed.

So my question is do I need to set a specific length for each pushrod and how do I go about that?

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Mk1: CKD 11/66 first registered 8/5/67; owned since 3/77. 18GB +40 balanced. Peter Burgess BVFR head. Piper 285. 123. FidanzaFW. 4synch c/r box. Lots more as I did a nut and bolt rebuild; finished 2015. Tartan Red.

GT: December '78. VW Golf guards, flush fit front and rear valances. Torana XU1 vents, frenched indicators & Mk1 rear lights. 'Worked' Rover V8 with Monsoon ECU for EFI. GM4L60E, Lokar tiptronic & Quick4 controller. Vintage Air A/C. FC IFS. CCE 4 link rear. Salisbury with Quaife. Jaguar Storm.



Edited 1 time(s). Last edit at 2014-04-23 08:40 PM by MGB567.

It is true that skimming the head will change the valve train geometry very slightly, but unless you are going racing or have skimmed the head a lot, I don't think that you will notice very much difference. Basically, having too long of a rod will cause (for a given clearance) the rocker arms to be at a slightly greater angle than normal. This translates into less vertical motion and more horizontal. But given that you usually skim a head by a 1/100" or so it won't reduce the vertical motion by very much - you can do the trig and figure it out if you are a keener.

I have seen that some of the hollow, light weight rods advertise themselves as being "slightly" shorter to compensate for head skimming.

I'm sure the racers and pros will jump in with a comment here.

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Terry Ingoldsby
terry.ingoldsby@DCExperts.com

As you skim material off of the head through a series of rebuilds, the push rod length needs to be shorter and shorter. If you ship up the rocker pedestals you need longer push rods. Since you've only shaved some off of the head, you're in luck. You can just screw the djuster on the rocker out farther to compensate for the missing material. You're going to be fine with stock push rods (and I stock the Smiths)until you start pushing very high rpms and very high valve spring pressures. Best of luck. Basil 707.762.0974 basiladams@yahoo.com

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Basil C. Adams
1956 MGA Coupe (Show Car)
1957 MGA Roadster (Driver)
1958 MGA Coupe (Racecar)
1959 MGA Coupe (unrestored)
1960 MGA Coupe (unrestored)
1960 MGA Roadster (Driver)
MKIII Elva Courier (E1056)
1967 427 Cobra
1972 Alfa Romeo Montreal
A coupla late MGBs
1960 Austin Healy BN7
More Cars than Brains

Barrie said, "... In it he notes that if the head has been skimmed you may need to adjust the length of the pushrod to achieve the original clearances ..."

I agree with Terry. If there are rocker arm push rod ball threads available for correct valve stem tip to rocker arm clearance (valve lash) there is no need to adjust the push rod length. Past that, the rocker arm assembly towers can be shimmed upwards to retain the stock length push rods.

There are experts on this site who know exactly how to adjust for cylinder head thickness, deck height, and camshaft lobe lift in order to maximize valve lift for all out racing engines. For street engines I don't feel that you will detect any difference.

Given the number if increased lift camshafts and increased ratio rocker arms being offered for sale by the usual vendors, coupled with head & deck surfacing, this deal can get real complicated.

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Be safe out there.
Kenny

What Basil said!

Have fun

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If ignorance is bliss, I must be the happiest man alive!
Signed

RUFUS
67 Roadster

I agree with the above statements, unless you have cut massive amounts off the head and block, for example in my race engine, the amount taken from the block and head add up to aprox. .160", add roller rocker arms to that equation, and everything that goes to getting them right, and yes I ended up using pedestal shims, and aftermarket non stock length push rods to optimize the geometry, and lift on my set up, on a street engine these amounts are much smaller, and while you might see a small gain a playing with this, it would not be enough to notice.

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Hap Waldrop
Acme Speed Shop
864-370-3000
Website: www.acmespeedshop.com
hapwaldrop@acmespeedshop.com

Hap, I finally figured it out. It required a magnifying glass but now I see that your dog is your head engine assembler. I thought you were just very hairy until today. I'm curious, with that much off the head and block, I assume you machine valve cutouts in the deck? I do this for a customer and I'm always concerned that we'll break through to water. Have you ever gone far enough for that to happen?

That's ridiculous, Dale. Everyone knows that all Hap's dog does at the shop is check endplay on crankshafts!

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Friends talking around a fire is the history of mankind.

All of the above x3 (except the part about Hap's dog... ).

The modest change in rocker arm geometry from a small cut on the head or block will not be noticeable on a street car...

Dick

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Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

Dale, FYI, the limiting factor on exhaust valve reliefs in the MGB block, is top ring position on the piston. Radial depth of the notch is a function of exhaust(only) valve size.

Dale what Dave said, I guess I probably cut them .100" before on a block that was deck additional .030" and never had a issue, most of these rish block have tons of deck thickness, for example on a 1275 SCCA race engine for years everyone ran a short deck Cosworth piston designed for the GT6 Mini with 6" rods that required you deck the block .250" on the Midget 1275 with 5.750 rods, and also I decked Triumph Spitfire block .250 before to run TR6 pistons in them. I've never went to such extreme on the B series block, but I it would not surprise if you could on them as well. I see you have a sonic tester have you ever done any testing on the MGB blocks?

Dale oh I see you have met ole "Mack the terrorist" Just click on my avatar pictures, and it will take you to my home page here and you see ole Mack up close. Here's a better look at him on a very "twisted" day, in fact I think he made this expression after reading a post here Oh and don't let me leave out ole Mo, the black lab, he is the king of goofiness They come to work with me each day, those freaking dogs have it made.

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Hap Waldrop
Acme Speed Shop
864-370-3000
Website: www.acmespeedshop.com
hapwaldrop@acmespeedshop.com

Yes, that's how I gage the depth, just shy of the top ring. I go a little larger than the valve od on the diameter, I have it written down somewhere how far I've gone. I've got one coming up to do, I'll take a video of it. I forget how long the program takes, I think it's 2 cycles of about 4 minutes each. A lot quicker than the old days on the Bridgeport.