clarkspeed Posted August 4, 2004 Share Posted August 4, 2004 Does anyone have any experience running a 38mm exhaust valve in a L6 head? I just built up a new head and decided to throw the 38's in there since I needed new valves anyway. I am running a high lift cam which throws them right into the cylinder walls. I know I need to "unschroud" the block to get a fit but it looks like a lot of material to remove. Curious if anyone else has gone through this before. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted August 4, 2004 Share Posted August 4, 2004 I've been running L28's with notched blocks for the last 10 years, but I'm not running a larger exhaust valve. I was just doing it for flow around that side of the valve. You can go up to the fire ring line on the headgasket, and I went about 75% of the way from the top ring to the top of the head. Never had a headgasket problem. I kinda wonder if you might get better flow out of a smaller valve with more room to flow all the way around it though... Quote Link to comment Share on other sites More sharing options...
DAW Posted August 6, 2004 Share Posted August 6, 2004 Since you "threw in" 38mm valves it implies that your engine had 35mm valves which came only on L24 engines (83mm bore). 38mm valves will crash into the tops of the cylinder walls on an 83mm bore unless you grind reliefs in the block to accommodate them. Nissan did provide a crude relief at the top of the L26 bores to accommodate the 38mm exhaust valves intefacing with the 83mm bore, but L24s didn't have this. I would assume L24E blocks have factory reliefs since they are 83mm bore and 38mm exhaust valves. Anyway, you can create a far better-flowing relief with a grinder creating a rounded relief than the factory does with their industrial/agricultural looking squared-off relief that they flycut into the top of the bore. DAW Quote Link to comment Share on other sites More sharing options...
JMortensen Posted August 6, 2004 Share Posted August 6, 2004 Uhhh, are you sure about that DAW? I thought your average L28 came with 35mm exhaust valves. How to Modify backs me up... Quote Link to comment Share on other sites More sharing options...
DAW Posted August 6, 2004 Share Posted August 6, 2004 You're right and I screwed up by relying on my memory instead of confirming by looking it up. I had a false memory of 35mm L24 and 38mm L28, when it's actually 33mm L24 and 35mm L28. I have to discipline myself that my memory is impaired because it just doesn't feel like it is to me. I was poisoned by CO due to a faulty furnace in a rental home back in '98 which resulted in permanent cognitive impairment due to toxic encephalopathy. Lawsuit against Prudential Property Management is taking forever but it's out there. Meanwhile, my career is trashed and I'm on disability. I've been a Zcar enthusiast for a long time and bought my first 240-280 back in '85 when I was in medical school. I guess I'm sharing this so that while I'm more often than not accurate, those who read my posts should take this as a disclaimer. I've known details about cars for decades with reliable recall and it still feels the same to me (I don't feel like my memory is bad but it is), but sometimes the recall is flawed since the CO poisoning. This is way more info than I should probably share here, but I want you all to know that you should double check info you get from any source before you treat it as fact. BTW, the concept is the same whether it's 35mm valves in an 83mm bore or 38mm valves in an 86mm bore, you have to make room for both the valve and the flow at the top of the bore and this is much easier to do on an engine that's been run (vs fresh hone or overbore) because the upper travel of the top ring is demarcated in the bore. If you go too deep down the bore the ring will flutter and fracture. Use the head gasket and ring mark as your guide. DAW Quote Link to comment Share on other sites More sharing options...
clarkspeed Posted August 7, 2004 Author Share Posted August 7, 2004 Yes, 35 is stock and 38 oversize, L28. Sorry about the brain damage. I think things will fit but I need to notch all out to the head gasket and down to the top ring. What I didn't realize was that it needed to be cut square to get the maximum amount of material out. A typical unschroud angle grind cut will not work. If I get it milled out it will look strange to me. I've never seen this amount of material removed before. Quote Link to comment Share on other sites More sharing options...
DAW Posted August 7, 2004 Share Posted August 7, 2004 Take a look at an L26 block. Maybe the factory square-cut approach isn't so crude after all because it does remove a lot of material. It just seems like a bevel would outflow a squared-off pocket. DAW Quote Link to comment Share on other sites More sharing options...
JMortensen Posted August 7, 2004 Share Posted August 7, 2004 I agree, but basically I just did as much as I could to mirror the head on the block. Nice rounded edges, etc. When I did the head, I set the headgasket on the head and traced it. I cut as close to the fire ring as I could. I did the exact same thing on the block. On an L28, there's really NOT that much material to remove. Not at all time consuming to get it done. Not like my old buddy's L16. We took LOTS of metal out of that one. Jon PS DAW--I hugged a tree last year at about 20 mph on my mountain bike and lost my short term memory for a week, so I can sympathize with your memory problem, although it sounds like mine was kinda opposite. I just couldn't put 2 thoughts together because I'd forget the second one before I said it. I wish you the best with that Quote Link to comment Share on other sites More sharing options...
DAW Posted August 8, 2004 Share Posted August 8, 2004 I've noticed that not all head gaskets are created equal and some are more true to the combustion chamber shape of the head than others and would provide a better template than others. PS-Jon, I did something like you did but I was one my road bike in the mountains in Utah. I never have had any recollection of the accident itself, just heading out on my bike for the ride that morning. The police assumed it was a hit and run. I was conscious that night but didn't know who I was and I had no short-term memory either but over the next couple of days in the hospital I started to recall things about myself and it seemed to come back in "packages". I recovered my memory within a week and it turned out that I was a medical student from Chicago who was doing a clinical clerkship in Ophthalmology at Univ of Utah and realized that I was absent from work on a different floor in the hospital that I was a patient in. I never have recalled the accident itself but my bike and myself were mangled. What I'm faced with now is different and to most people who don't know me probably think I'm normal because my intellect is intact. Even though I'm not walking around mumbling and drooling, my "excutive function of cognition" specifically is wiped out and it's like I'm smart and stupid at the same time. Life goes on and I try to make the best of it because it's permanent. DAW Quote Link to comment Share on other sites More sharing options...
JMortensen Posted August 8, 2004 Share Posted August 8, 2004 I've noticed that not all head gaskets are created equal and some are more true to the combustion chamber shape of the head than others and would provide a better template than others. I was thinking about this last night. I used a Stone gasket because they've been pretty universally available and always very good quality. Now I'm thinking that if I used an HKS metal gasket for the stroker buildup I could have done more unshrouding and more notching on the block, and gotten presumably better flow out of the head. It does have larger diameter bores, right? In a case like clarkspeed's that might be just the ticket... Quote Link to comment Share on other sites More sharing options...
clarkspeed Posted August 12, 2004 Author Share Posted August 12, 2004 I have the HKS now and it does have a large bore. It will give me the side clearance I need if I take it to the limit. Damn near 1/8" on the radius. If I make a square notch down to the top of the ring it's going to look pretty radical and will cost me a few compression cc's. Anyway, it's more than I can do with a hand grinder and will need to be milled. Are you running a stroker now? I've got a diesel crank on the bench I've been thinking about using...... My question is: What kind of RPM is practical for one of those things. That would determine my cam selection. Quote Link to comment Share on other sites More sharing options...
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