Printable View
Greeting friends, I am just replacing the cylinder head on my 92 Previa with a rebuilt one. The exhaust cam on the exchange head has an extended snout which I assume is from the Super Charged Previa. Mine is non S/C. I don't have the lift specs for the head. My questions; 1) Are the cams interchangeable? 2) If not, any thoughts about transferring the cams from my old head given that they were not machined or run in on the exchange head. Many thanks!
Hello Alberta, welcome to TVT!
All cams for all years are the same for all NA (Naturally Aspirated) Previas. Although the head castings are the same for SC (Super Charged) Previas, they use different cams than NA Previas. Among the SC Previas, there were changes in the cams between 94-95. If you could verify the head came off a NA Previa, then I would use the cams that came in it. Since you don't know, unless you have a way to verify lift and duration, I would stick with your original cams.
I wouldn't worry too much about the old cams mating to the bearing surfaces of the replacement head. These surfaces are precision machined and not likely to pose any problems. If you have doubts, use PlastiGauge (available at Napa or other parts stores) to verify clearance.
Keep in mind that all 16 valves are adjusted individually with shims. There are many different shims of varying thicknesses available through Toyota. Since you are swapping cams you will need to go through the adjustment procedure for each valve. Although it probably doesn't matter much, keeping location in perspective, I would probably move all the shims from your old head to the new, then install the cams. Position the intake cam to TDC (put the marked sprocket tooth straight up to the 12 o'clock position) then verify the marks on the intake & exhaust cam gears line up. Using feeler gauges between the cam lobes and the shims, determine lash, or clearance (you can only check half). After measuring and recording, rotate the intake cam sprocket 180 deg (marked tooth 6 o'clock position) and repeat on the other half. Intake valves should have .006 - .010 in clearance, and exhaust valves should be .010 - .014.
In order to adjust, you need to remove the shim and use a micrometer to see how thick it is. Then based on your initial measurements, you will need to add or subtract to determine the new shim size. Example: If you measured .018" clearance on an exhaust valve, (.006" larger than the desired .012"), you would take the existing shim thickness (say .114"), add the .006" to it, buy a new shim (.120") from Toyota, and install it in place of the old. Since you have a whole other set of used shims, you might check those and save a few $$$ (assuming you're lucky enough to find the correct sizes).
Since your head is off and will be on the bench, I'd probably just pull the cams back off for access when you install the correct shims (cams need to come back off before installation of head anyhow). I'd recommend double checking lash after head is installed and torqued. If by chance you find a problem with lash after final installation, there is a special tool to remove shims and reinstall (with cams on). It's Toyota Part number SST 09248-55040 and it's available through Kent-Moore tools at 800-345-2233. Price is about $75. Note: These clearance/lash dimensions are for cold engines. If you recheck after the project is complete, make sure the engine is stone cold. Tim
http://i8.photobucket.com/albums/a23...v/IMG_7411.jpg
Tim, thank you very much for the detailed reply. You will have saved me a fair amount of grief! I will transfer over the cams from the old head to the new and go through the dance on the value adjustment. If the valve is too tight, any thoughts about taking off a bit from the top of the valve stem vs. hunting down the right shims? I will have all the shims off the old head that might give me some choices to work with. Just wondering...........Toyota should be sending your a marketing commission, you are a real ambassador for the brand. Cheers- Chris
Removing some material from the valve stem is an alternate way to get it done, but just swapping shims is easier (assuming you have the right ones available). I believe Toyota uses these same shims in other applications, so I think it's likely your local dealership would have most sizes in stock. If not they would probably only be a day or so away. Tim
Finished up the job using my old cams to avoid the SC mix up and adjusted using the shims. I also did a very light rebuild and ball honed the cylinders, replaced the rings and rod bearings, water pump etc. With 500,00km on the engine, the old bearings showed very little wear, but given the low price I replaced them anyway. The rebuilt head was from Clear Water in Fl. The head was clearly an aftermarket casting that was new and while the quality doesn't measure up to an original Toyota casting, it will do the job fine. Clear Water shipped promptly and I am satisfied with their service and price ($419 plus shipping to Canada. I did the job over three weekends and was careful to clean and inspect parts carefully and label everything. The van fired up and ran perfectly. Was it worth the time and effort? For about the cost of two months payment on a new car, I should get another 200,000 to 400,000km out of the engine. Since my time was weekends and I couldn't have billed anyone for my time, I guess the combination of my time which I don't pay tax on and the satisfaction made the job worth it. But I have a well equipped shop, good tools and while not trained as a mechanic, I have been turning wrenching with my dad my entire life (He is 85 and we still have our best visits when he brings his car over and we work on it together). Sure, it was worth it, but it is a big job if you don't have a hoist and are crawling around under the van with 500lbs of engine and trans 8 inches from your face. Tim, many thanks for your help.
I like stories like this. Thanks for sharing. Tim