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GM BBC 990's questions

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GM BBC 990's questions

Postby e-tach » Sat Dec 20, 2008 1:34 am

Working on a pair of 990 rect. port heads. They have had numerous valve jobs on the 2.25/1.88" seats. Is it possible to take them to 2.300"/ 1.900"? At quick glance, it seems with the factory guide spacing it shouldn't be an issue. Bore size is 4.350"

At the same time, would like to upgrade to 11/32" stems. To the same rules apply to guide removal/installation as othe OEM BBC heads, as far as which way to drive em in/drive em out? Or would it be best to just machine them for .500" guides?
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Postby ProPower engines » Sat Dec 20, 2008 1:44 am

If you have to remove them drive them out from the valve head side to the rocker arm side.
I don't know if anyone make a 11/32" replacement guide as I had that same job to do a while back and replaced the guides and pressed in solid bronze .500od/.342id guides and finished them to size.
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Postby hsutton » Sat Dec 20, 2008 1:58 am

Wouldn't putting in bronze liners be the best and least trouble free method of shrinking the 3/8" guides down to a 11/32" size.
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Postby needforspeed66gt » Sat Dec 20, 2008 2:38 am

Kline makes the conversion liners for going down to 11/32" - that would be my choice.

I also would just put new seats in the heads and stick with 2.25"/1.88" valves.
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Postby Keith Morganstein » Sat Dec 20, 2008 6:35 am

hsutton wrote:Wouldn't putting in bronze liners be the best and least trouble free method of shrinking the 3/8" guides down to a 11/32" size.


Yes. that work's. Knurl the 3/8's existing guide and then Install a 11/32's bronze guide liner.

Guide liners or conversion guide through the existing guide.

Replacing the factory guides (press out and in) sucks. The guides are stepped and only go one way. You can get into trouble and ruin good heads.Then the valve job after.... way out.
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Postby leahymtsps » Sat Dec 20, 2008 7:32 am

Personally I would install Kline guide liners and replace the seats and stay with the same size valves.

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990 Head Repairs

Postby GOSFAST » Sat Dec 20, 2008 8:53 am

If possible leave all the existing guides in place and work with them as-is!

This will save much aggravation/labor!

Your first choice is the best, install .500" guides inside the OEM's. This keeps the seats in a relatively "unmoved" position!

In order to "step" the .375" guides to .343" (with liners) you need the .375" guide in "new" condition. Too much involved repairing those first then stepping them down! Easier to install the .500" pieces and with this procedure you have the .343"/.375" option either way with one operation!!

Personally, I would leave the .375" UNLESS you need to purchase new valves, if so, then go to the .343" platform. My own personal preference also would be "cast" guides only, no "bronze" unless the valve lifts are high! Factory "cast" guides get the job done in most cases! Have for years now!

The 2.300" valves will go, along with the 1.880" if necessary.

(Add) Only the OEM exhaust guides must be removed from the bottom side of the heads "out" towards the rocker side and installed from the tops. They are stepped, as mentioned above, and "water-cooled". They "close-off" the cooling system between the upper and lower sections of each individual guide. If you have a new exhaust guide handy you can measure the two steps.

Thanks, Gary in N.Y.

P.S. Just delivered a unit (509") with those 990's. Heads flowed 350 in and 244 out. These had 2.190's, 1.880's and .375" stems! The .343' stems would have added 4-5 cfm (nom.) to the numbers!
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Re: 990 Head Repairs

Postby needforspeed66gt » Sat Dec 20, 2008 2:55 pm

GOSFAST wrote:In order to "step" the .375" guides to .343" (with liners) you need the .375" guide in "new" condition. Too much involved repairing those first then stepping them down! Easier to install the .500" pieces and with this procedure you have the .343"/.375" option either way with one operation!!!


You don't need to have guides in "new" condition - you bore the .373 guides to size for liners @.433 or whatever that # is, then use K-Line's .060" wall thickness liners which bring you down to .341
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Re: 990 Head Repairs

Postby GOSFAST » Sat Dec 20, 2008 4:20 pm

needforspeed66gt wrote:
GOSFAST wrote:In order to "step" the .375" guides to .343" (with liners) you need the .375" guide in "new" condition. Too much involved repairing those first then stepping them down! Easier to install the .500" pieces and with this procedure you have the .343"/.375" option either way with one operation!!!


You don't need to have guides in "new" condition - you bore the .373 guides to size for liners @.433 or whatever that # is, then use K-Line's .060" wall thickness liners which bring you down to .341


To each his own! Everyone has their own way of repairs, I would just add this for what it's worth. I wouldn't be too comfortable with a .433" hole and a .500"/.530" positive type seal?? Don't even like the .500" seals with the .375" stems, prefer the .530" instead!

Risky at best!! But would depend on the seal used? You could go to a larger seal, leaving more of the original guide, but then the springs may become an issue!

I'll go back to what I put up before, you try to keep the labor/man-hours at a minumum and get the job done!

With the .500" O.D. guides you may even get by without machining for the seals, assuming a .500" seal is in the program! We have these particular guides on the shelf with the liners already installed if you choose the bronze-lined deals. Either for the .343" or the .375" stems. Install the 16 guides and your done!

To me it seems the "way-to-go", but as I stated, to each his own!!

Thanks, Gary in N.Y.

P.S. I prefer to stay clear of .060" liners, just my own preference here!
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Postby needforspeed66gt » Sat Dec 20, 2008 7:40 pm

I use a .562" seal leaves plenty of meat up top and clears double springs.

Like Keith said, using full replacement guides is dangerous and can cause leaks to the top side on the water cooled exhausts if you pull metal on removal. I have done the .502" full bronze inside the OE guide countless times - I have also spot-faced down the OE guides to install hatted .502" guides - both work fine. The .060" liners are a cheaper alternative with no downsides that I have ever seen if installed correctly just like any other liner.
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Postby e-tach » Fri Dec 26, 2008 10:32 pm

Working on a pair of 990 rect. port heads. They have had numerous valve jobs on the 2.25/1.88" seats. Is it possible to take them to 2.300"/ 1.900"? At quick glance, it seems with the factory guide spacing it shouldn't be an issue. Bore size is 4.350"


I have attached a pic of one chamber, so you could see what the old valve job looked like. Surprising the engine ran as good as it did. I dont want to question anybody here - just wondering why the 2.300"/1.90's would not be a good idea here, due to the sunken seats.

Image
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Postby maxracesoftware » Fri Dec 26, 2008 10:51 pm

its not that bad of work !

the exh side doesn't look too bad.

the intake VJ doesn't have enough bottom angle width all the way
around

needs a lot of chamber floor or roof blending
(depends on your Viewpoint orientation)
into the valve job-to-chamber transition
and i'd go with 2.300/1.880 or to 2.300/1.900

just because its sunk a little, does not mean it can't be
successfully blended back into a nice transition

you still have plenty to work with
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Postby e-tach » Fri Dec 26, 2008 11:02 pm

Thanks Larry-

Yes - I planned on blending everything in. And I also see what you are saying about the lower cut.

The heads currently have 2.250/ 1.880's. So yes, I would like to go to 2.3/1.9.

I am a bit leary of the 2.300 intake seat though - it appears the heads have been milled quite a bit, not sure if I will be able to get a good enough 45 cut up by where the chamber meets the deck.

How should I approach the de-shrouding operation?
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Postby maxracesoftware » Fri Dec 26, 2008 11:16 pm

e-tach wrote:Thanks Larry-

Yes - I planned on blending everything in. And I also see what you are saying about the lower cut.

The heads currently have 2.250/ 1.880's. So yes, I would like to go to 2.3/1.9.

I am a bit leary of the 2.300 intake seat though - it appears the heads have been milled quite a bit, not sure if I will be able to get a good enough 45 cut up by where the chamber meets the deck.

How should I approach the de-shrouding operation?



don't be afraid to "sink" the Intake VJ enough to get
a full width 45deg seat angle....and some "extra" depth to get
at least .020 or .030" top angle cut width above the 45deg
at the seat-to-deck interface

that will give you a little left if you need
to take a lite resurface off the heads later on

also do the intake valve job 1st
then do the exh valve job last
after you establish the depth of the intake seat 1st

nothing wrong with a little "sinking" of the valve job
as long as you know how to regain a blended transition
back into the Chamber
Last edited by maxracesoftware on Sat Dec 27, 2008 12:48 am, edited 1 time in total.
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Postby e-tach » Sat Dec 27, 2008 12:03 am

Thanks again Larry - all very very good infomation, well put - and greatly appreciated.

Any advice on deshrouding or chamber reliveing for the bigger valves?
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