Rear Head Question

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silverstrom
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Rear Head Question

#1 Post by silverstrom » Thu Jun 02, 2016 1:13 am

Why is there a difference between the rear head machining and the front head machining? If you look at both heads it looks like the rear head has been milled. There is very little height difference between the combustion chamber and the head surface. The front has a very noticeable step, but on the rear it's nearly flat. Why is that? The combustion chamber volumes look to be the same, so is the difference in machining to increase compression slightly in the rear cylinders?

I checked several sets of heads here and they are all the same. I never noticed this before, but I wasn't really looking for it either. I just assumed that both heads were profiled the same. They are not.

Ron57
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Re: Rear Head Question

#2 Post by Ron57 » Thu Jun 02, 2016 5:08 am

Hi I'm just putting a motor together and checked the squish, head clearance and found I had to take
20 thou off the front head so they will match. Probably mixed parts.
Cheers Ron.

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phildu31
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Re: Rear Head Question

#3 Post by phildu31 » Thu Jun 02, 2016 6:39 am

Hi

Compression ratio has been modified in 1GE specs and heads have a different part number vs 47X.
Ratio was 6,6:1 in all cylinders in 47X.
But lowered to 6,4:1 on upper cylinders and 6,0:1 on lowers.
Seems you have 1GE heads.
Anyway I have no idea how to recognize it, maybe some mark different anywhere.
Do you have any pic ?

Phil.

silverstrom
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Re: Rear Head Question

#4 Post by silverstrom » Thu Jun 02, 2016 12:32 pm

Ron57 wrote:Hi I'm just putting a motor together and checked the squish, head clearance and found I had to take
20 thou off the front head so they will match. Probably mixed parts.
Cheers Ron.

It isn't mixed parts. I've checked a box full and they are all like that. I talked to someone who checked theirs and found the same thing. It's supposed to be like that, but why?

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Re: Rear Head Question

#5 Post by silverstrom » Thu Jun 02, 2016 12:48 pm

phildu31 wrote:Hi

Compression ratio has been modified in 1GE specs and heads have a different part number vs 47X.
Ratio was 6,6:1 in all cylinders in 47X.
But lowered to 6,4:1 on upper cylinders and 6,0:1 on lowers.
Seems you have 1GE heads.
Anyway I have no idea how to recognize it, maybe some mark different anywhere.
Do you have any pic ?

Phil.
Milling the head shouldn't lower compression, it should raise it. The volume must larger in the upper combustion chamber. That would lower compression. But why mill it afterwards? To improve squish with the higher cc head?

If what you say is true I'd have to say that this would all be to deal with higher than expected operating temperatures in the rear cylinders of 47x bikes and Yamaha made changes to the 1GE rear head in an effort to lower the rear cylinder temperatures.

I can't find any information on the head volume in the 500 manual. I'd like to know the volume of the 47x head vs the 1GE head. They must be different based on your information. Anyone have details on the head volume?

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phildu31
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Re: Rear Head Question

#6 Post by phildu31 » Thu Jun 02, 2016 1:16 pm

Yes, milling the head will increase compression.
I've no idea why Yamaha lowered more compression on lower cylinders.
Image
Logically they should have lowered more compression on bottom heads due to higher temperature in cylinders ( coolant flow design ... ).
One moment, I thought there was an inversion in the service information, but what you noted by yourself tends to confirm it.
When my 47X heads were off, I don't remember having seen any difference between the 2 heads.

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WVWRZ500N
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Re: Rear Head Question

#7 Post by WVWRZ500N » Thu Jun 02, 2016 1:19 pm

The 1984-85 models were cast and then machined leaving approximately a .020" step in the lower cylinder head where the deck meets the edge of the squish band. Compression ratios were as stated above. The RZ500S model 1986 engines had the lip effectively removed which equalized the ratios upper to lower from the factory.

When I build the engines now all 4 are equalized even on a blueprint rebuild.
Regards,
Bill Wilson
Wilson Performance
Lee's Summit, MO
http://www.wilsonperformance.net
Member RZ500 Owners Group #573
816-377-3185
WVWRZ500N@aol.com

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phildu31
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Re: Rear Head Question

#8 Post by phildu31 » Thu Jun 02, 2016 1:39 pm

Beware that the 47X heads have the same compression at 6,6:1.
Will scan the corresponding page of service information later as I can't find it on rzv500r.net
So Yamaha went back in 86 !
I'd like to know how to externally recognize which type of head, given that they have different parts numbers ( 47X vs 1GE ).
Maybe some type of marking ? Any pics ?

silverstrom
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Re: Rear Head Question

#9 Post by silverstrom » Thu Jun 02, 2016 2:41 pm

phildu31 wrote:I'd like to know how to externally recognize which type of head, given that they have different parts numbers ( 47X vs 1GE ).
Maybe some type of marking ? Any pics ?
I have a spare complete 47x engine. I'm going to check the heads and compare to the box of 1GE heads I have to see if there is any way to distinguish between the 2 from an external point of view.

I think it's safe to say that most people have no idea there is a difference and have been using whatever heads they have with no ill effects. The fact that Yamaha went back to the 47X head in 86 proves the change was pointless.

I have a hard copy of the 86 RZ500S manual from Yamaha and checking it now shows that both upper and lower heads are back to 1985 47X part numbers. That effectively means we can ignore any changes that happened in 85 and use whatever heads we have on hand.

Bill's approach and matching the heads seems like the logical choice during a full rebuild. For those of us doing minor work at home I think using whatever heads you have on hand should be just fine given Yamaha's decision to ignore the 1985 change and go back to 4x heads in 1986.

Yamaha did something similar with the RZ350 heads, using the Y1 and Y2 head. The later RZ350 1UA cylinders had different porting (raised) and the Y-2 head had a lower CC volume to maintain overall compression due to the raised ports. That means that you shouldn't use the Y-1 head with slightly larger cc volume with the 1UA cylinders as it would have the effect of actually lowering compression slightly. However, in practice, many are interchanging the RZ350 Y-1 and Y-2 head without knowing they have different cc volumes and no one is complaining about performance issues or ill effects.

There is also a Y-3 head for the RZ350 and while I did spend a lot of time searching for more information on it, I was never able to find anything conclusive. My own experience with the 350 tells me the Y-3 head was for the last run of 350s produced in Brazil where fuel quality was so poor that a higher cc volume was needed to prevent detonation. I actually had some Y-3 heads here and should have checked the cc volume myself, but they were shipped off for machining before that could be done. A bad decision on my part as it would have answered the question once and for all.

So, Yamaha has a history of changing head design on the same bike from one model year to the next, although many people, if not most, don't know about it.

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phildu31
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Re: Rear Head Question

#10 Post by phildu31 » Thu Jun 02, 2016 3:34 pm

I think you've got it : Y-1 and Y-2 marks !
I already noticed this on barrels and heads but didn't know more, mine have the Y-1 mark.

silverstrom
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Re: Rear Head Question

#11 Post by silverstrom » Thu Jun 02, 2016 10:31 pm

Every head I've seen has been Y-1. Has anyone seen a Y-2 500 head? Any photos?

I checked my spare 47X motor, but both heads have been milled. The casting marks are the same as the 1GE engine. 47X1 and 47X2. Y-1.

I checked 16 cylinders and 8 heads and all were the same. No Y-2 there.

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Re: Rear Head Question

#12 Post by tacky1 » Thu Jun 02, 2016 10:52 pm

silverstrom wrote:Every head I've seen has been Y-1. Has anyone seen a Y-2 500 head? Any photos?

I checked my spare 47X motor, but both heads have been milled. The casting marks are the same as the 1GE engine. 47X1 and 47X2. Y-1.

I checked 16 cylinders and 8 heads and all were the same. No Y-2 there.
That is quite the collection John... You can send some on over my way if you get bored looking at them.. :mrgreen: :mrgreen: :smt005 :smt004
1985 RZv500
1984 RZ500 Hybrid
1986 RG500 Walter Wolf
1986 RG500 Skoal Bandit
1984 RZ350
1984 RZ350 Hybrid
1981 RD350LC
1981 RD350LC Hybrid
2009 CR500AF Supermoto 250X
2007 CR500AF 250X
1988 YSR50 (2)
1989 VFR750R RC30
1984 GPZ750 Turbo

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WVWRZ500N
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Re: Rear Head Question

#13 Post by WVWRZ500N » Thu Jun 02, 2016 11:50 pm

I've built near 40 engines in the past 15 years and have never kept track of 47X vs. 1GE vs. whatever. But, the fact is the early head were all different upper to lower. The ONLY engines with corrected compression from the factory was the 1986. I've built two 1986 engines. Besides the crank differences the balancer was driven from the upper crank from the factory as well.

Somewhere I have a comparison of the earlier models and the 86 from Yamaha. It too detailed all the changes and stated specifically that compression ratios were corrected.
Regards,
Bill Wilson
Wilson Performance
Lee's Summit, MO
http://www.wilsonperformance.net
Member RZ500 Owners Group #573
816-377-3185
WVWRZ500N@aol.com

silverstrom
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Re: Rear Head Question

#14 Post by silverstrom » Fri Jun 03, 2016 12:09 am

tacky1 wrote: That is quite the collection John... You can send some on over my way if you get bored looking at them.. :mrgreen: :mrgreen: :smt005 :smt004
I'm not bored with the view yet Paul, but I'm getting there. I may move some stock soon.

Image

silverstrom
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Re: Rear Head Question

#15 Post by silverstrom » Fri Jun 03, 2016 12:34 am

WVWRZ500N wrote: The ONLY engines with corrected compression from the factory was the 1986. I've built two 1986 engines.
That's interesting, because the part numbers for the 86 heads are the same as the part numbers for the 84 heads. They changed in 85 but for the S model went back to the 84 #s.

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