Piston to Valve Clearance Question

[fiveironbiff]

I have to buy a new set of rocker arms for my 89 LX and I just wanted to ask if anyone would be able to tell me if I'll have any clearance issues with the Cobra 1.7 rockers. I currently am running the B303 cam with a duration of 224°/224° @ .050'' and a lift of .480''/.480''. The piston are TRW 9:1 with valve reliefs. I do not know what size the valves are though. If there's a way anybody could make an educated guess as to if I'll have any problems with that combination, please let me know. Thanks for any help

 

[fiveironbiff]

Anybody? I'm needing to find out asap because I can get a set of the 1.7 Cobra Rockers today, or I have to let my local speed shop know to order the 1.6's today so they'll be here in the morning

 

[redterror302]

The only real way to determine it is by measuring. I know it sucks, but there is no way to be sure of what kind of clearance you have, especially if you do not have stock pistons/valves. What kind of heads do you have, I am sure someone will know what size valves they are. I know a lot of people that run a b cam with 1.7's, but I think they all have stock pistons and valves. Do yourself a favor, be on the safe side and check it properly.

By the way, would you be purchasing the 1.7's used? If so, I am sure you would be able to sell them if they wouldn't happen to fit.

 

[fords2fast4u]

YOU never said what heads you have - so i will asume they are stockers... i see no problem with clearance on the stock 1.84's or whatever they are, i have forgotten. I would run it... my buddy with an 88GT with 2.02's and more duration and lift has no problem. He has speedpro pistons, but i can't see the valve reliefs being that far off from one another.

 

[fords2fast4u]

5.0GT has a point, but i am under the impression that the motor is together (heads on)...

i would want to measure in his situation, but i would be able to live without it, considering the #'s = max lift on the stock stuff is somewhere in the mid 5's - i don't think he's pushing the envelope

 

[redterror302]

i would want to measure in his situation, but i would be able to live without it, considering the #'s = max lift on the stock stuff is somewhere in the mid 5's - i don't think he's pushing the envelope

That is an assumption that cannot be made. When the valves are at max lift, the piston is way down in its stroke, creating no problems at all. It is between 30 degrees BTDC and 30 degrees ATDC where you will experience your problems. Max lift does not matter there, so you cannot assume that just because a cam has .5xx lift that you will be fine. You can have much lower max lift than that and have problems with piston to valve clearance. It is all about the duration of the cam and the ramp rate at which the valve opens/closes.

If you do have the heads on, there is still a way that you can check your piston/valve clearance. Read up here: http://www.geocities.com/jjonibones/PVC.html

Assuming you do have stock heads and valve reliefs on the pistons that fit the stock valves, you can do the drop valve method and be done with it. If you do not have stock valves or valve reliefs that fit your valves, you are going to need to do the clay method to ensure that you have enough room between the edge of your valve and the valve relief in the piston.

All of this has to be taken into consideration, because you already stated that you do not have stock pistons, and you do not know what size your valves are. Just because so and so had this combo and it fit, does not mean that yours will too. It is not worth the risk to your engine to not check it.

 

[fiveironbiff]

I do still have the heads on the block in the car. I just spoke with the last owner (nearly impossible to get ahold of) and he said that the valves are 2.02 and 1.60 and when he put the motor together he used the clay method to determine valve clearance and had plenty with the 1.6 ratio. I greatly appreciate your responses because I seem to learn something new every time. I'll probably end up pulling the heads off so I can do things right and slow down and not rush like I've been trying. It's worth the time now to do it right and not end up with a damaged motor. Again thank you very much for your responses and that link!