Swapping heads

[crawdad]

Found some galling in two of the camshaft bearing journals on my 81 Special. I have a '79 model head that looks great. Was wondering if there would be any problems putting this head on my 81. I know the carbs were set up different on these models and I think the 79 may have had smaller intake or exhaust valves. Would appreciate any feedback from those in the know.
Thanks.

[TopCatGr58]

Re: Swapping heads

Originally posted by crawdad
Found some galling in two of the camshaft bearing journals on my 81 Special. I have a '79 model head that looks great. Was wondering if there would be any problems putting this head on my 81. I know the carbs were set up different on these models and I think the 79 may have had smaller intake or exhaust valves. Would appreciate any feedback from those in the know.
Thanks.

I wouldn't say that I'm totally in the know, but according to my manual, which covers 78-81 models, there is no mention of any differences with regards to the heads, valve train. In 82 when they added the YICS there were some changes made, but I think you will be okay. The manual describes using plastigage to check the cams and bearing caps for proper tolerances. Good Luck.

[sixtysix]

The way I understand it the 80 combustion chambers were changed in shape from the 78-79, due to emissions. The shape change lowered the compression ratio slightly. Do not know this for sure only what I have read and heard. I think the swap would work though, don't think pistons were changed, but havn't checked part numbers.

[crawdad]

I think your right about the combustion chambers being changed. I've been doing a little looking and measuring and I can't find any difference in the valves themselves. As far as pistons go, I have no way of checking part numbers, but the domes look exactly the same. Bore/Stroke are same specs. Oh well, there's only one way to find out, right? I'll let you know how it turns out.

[Mike Hart]

compression ratio

This is an area that needs some clarification. At The Deuce, Denny said he has had 78, 79, and 80 heads in his hands and can see no difference.

It was generally agreed that the compression ratio change is due to a gasket thickness change (thicker on the 80 and up).

I've heard stories about smaller valves, but Denny said not true in 80. Maybe 81? Anyone have hard info on that?

[JP]

80 was a transition year for these bikes and I believe that some of the early 80 bikes may have had leftover parts from 79's installed. I remember Zoltan's bike was quite the amalgam of 79 & 80 parts.

I can't really back this up with factory data, just from observations of other owners.

[Mike Hart]

valve sizes

Okay, digging around spec pages, I have found that in 1980 the valve sizes were indeed changed.

78-79
INT 36mm
EXH 31mm

80-82
INT 38MM
EXH 32mm

1 or 2mm is hard to notice visually without holding valves up to each other or measuring them...

[crawdad]

Admittedly I was using an old rusty caliper to make my measurements. Hard as heck to read. Assuming Mike is correct with the specs he has located, how would this affect the swap (other than possibly increasing compression a little)? How about carburetion? Bound to change that some I suppose, huh? Any other advisories before I waste "another" head gasket?

[TopCatGr58]

Originally posted by crawdad
Admittedly I was using an old rusty caliper to make my measurements. Hard as heck to read. Assuming Mike is correct with the specs he has located, how would this affect the swap (other than possibly increasing compression a little)? How about carburetion? Bound to change that some I suppose, huh? Any other advisories before I waste "another" head gasket?

Looks to me that if you use the smaller valved head that you might possibly loose compression due to smaller inlet's for the fuel/air mixture to enter!?

But, now I'm really going to show my mechanical ignorance.... if the galling is relatively mild, (isn't galling like a scratch in the metal from a foreign object ie. piece of metal, grain of dirt, etc?) and after disassembling, cleaning, the foreign matter is gone, won't the oil still work it's way around the camshaft, with just a little bit more in the galled groove, but otherwise still be okay??

When I did my rebuild I think I found a little groove in one bearing cap as well, but since these engine/parts are so hard to come by, I just cleaned it up and put it back together, that was 2 years ago, and a ~1,500 mile trip to NY and back from Va. with no signs of failure!!

I have a spare "NEW" OEM head gasket for the 81SH that I got prior to realizing I needed oversized one with my piston kit, couldn't return it to YAMAHA, let you have it cheap, new is $85.00, a $20.00 will due, with S & H.

[crawdad]

Check your mail TopCat.

[crawdad]

With some of the stupid questions I've plastered all over this board, my mechanical genius has already been divulged for all to see. So what's one more stupid question?
I thought, however that the whole intent of the change in valves was to "lower" compression for use with lower octane fuel. I would think (probably wrongly) that the increase in valve size would affect the volumetric efficiency in a way that could possibly achieve the effect they were looking for. Heck I'm just stabbing at flys here with this theory though. Perhaps some of our more astute engineer/mechanical types can help us out here.

[Mike Hart]

all XS and XJ models were designed for regular unleaded fuel.

Valve size has nothing to do with compression, as far as I know - when the valves are closed (compression stroke), their size is irrelevant, yes?

At the same time the valves were made larger, the carb jets were made smaller. Would someone please explain why these two things go together?

Emissions standards were the driving force here, I'm sure, as the 78E was the fastest production XS - future changes were not for power, but lower emissions.

[crawdad]

Yep, you're right. I was thinking "lower emissions", but that ain't what came out. A change in "combustion chamber" (like has been mentioned), as well as a thicker gasket, would change the compression ratio somewhat I believe, but not valve size. You're right.
Someone else (locally) told me to try using the head in spite of the galling. He too, had reassembled one that had some galling in the cam bearing journal, but said he couldn't tell any difference when he put it back together and has been running it for several years. I'm sure it couldn't have been toooo bad though. Metal shavings and engine parts don't get along too well from my understanding.