5/16" pushrods are standerd, but you might have to have a machine shop install them... to use them you will have to pull out the press-in rocker studs and use the screw in type.... why do you need them, for a stock application you wont need them, im running comp cams guide plates, but im also running a half-inch lift cam and higher rpms.. also if you use guide plates you will need to get the harder forged steel pushrods, or else the hardened steel guidplates will ware metal off the pushrods causeing inevitable failure....
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George Thompson
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65' Mustang Fastback - 289 with a hot rebuild, 4 speed close ratio toploader, 3.55 gears
You have to use guide plates on the heads that have the "slots" or "oval" holes for the pushrods. If the holes are circular, than it does not require a guide plate because the head itself acts as a guide plate. Or so I'm told all of that by a knowledgeable Ford guy.
My big question is, do I need guide plates or not? It seems from the sources I run into that all guide plates require screw-in studs(I don't know how much truth there is to this)? But if that's the truth, did my heads not have guide plates from the beginning? Because they have the press-in studs.
So I guess the big question, is did 66 289's have guide plates?
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Gasoline is for washing parts.
Alcohol is for drinking.
Nitro is for kickin ass!
1967 Mustang Coupe
R.I.P. Darrell Russell 1968-2004 The racing world has lost a great racer.
nope the origenal 289 head didnt come with guide plates. and if you just plan to do a decent rebuild and dont plan on having the engine see high rpms there is no need to have the heads rebuilt with screw in studs.. and yes guide plates require screw in studs. the reason for guide plates is to prevent the pushrod from bowing while running at high rpms 6500+.... the 289 heads came kinda pre guide plated, if you look the pushrod will sit close to the head on both sides., but the head is made out of soft metal and that is why racers use guide plates because they are made out of stronger metal, but intern that requires harder pushrods too. i hope this helps..
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George Thompson
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65' Mustang Fastback - 289 with a hot rebuild, 4 speed close ratio toploader, 3.55 gears
Alright, maybe I won't put the guide plates in then. But I will most likely have my shift point set somewhere around 5500 or perhaps a little after. But that's only the track that I will wind it up that much.
__________________
Gasoline is for washing parts.
Alcohol is for drinking.
Nitro is for kickin ass!
1967 Mustang Coupe
R.I.P. Darrell Russell 1968-2004 The racing world has lost a great racer.
Early 289 heads had "oval or slotted" pushrod holes and "non-rail" type rocker arms while later 289/302/351 heads had round pushrod holes and "rail" rocker arms. The slotted pushrod holes in the early heads acted exactly like a guide plate, it kept the pushrod and rocker arm aligned to the valve stem by limiting the movement of the pushrod. The later engines acheived the same goal of maintaining alignmnent by limiting the movement of the rocker arm, hence the "rail" on the arm and the longer valve stem length above the spring retainer. The late model pushrods are not hardened, while the early model pushrods are, since they rub on the slot in the head. Later on, Ford went to a completely different rocker arm which used a "sled" shaped fulcum (rather than the "ball" fulcum in the early rockers) which provided the alignment without the long valve stems or pushrod guides, except when solid lifter cams were used (i.e. Boss 302, 429 SCJ etc).
If you have early heads with pushrod slots, there is no reason at all to have guide plates. The slots provide the same alignment capability as the plates. This is what allowed use of the solid lifter cam in the 289/271 engine, which had slots, "non_rail" rockers and no guide plates. If you have later model heads with pushrod holes, and do not use the "rail" type rocker arms, you have to use some sort of guide plate to keep the rocker arm aligned with the valve stem. If you stay with "rail" rockers, then you do not need any guide plates.
Stock 289 pushrod diameter? 
