Setting compression after CC'ing heads.

[harpwrench]

I figured it with a .040" gasket at my elevation

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[Buglet]

   I'm throwing this out there if this would be a option. I know this is going backwards but couldn't he open out the chamber in the heads to get his cc's back.   

[billbuilds]

I figured it with a .040" gasket at my elevation

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     Correct me if I'm wrong but I don't see where you figured for the -4 degree cam gear. Also, it's my understanding that you leave the **intake close at zero if you've selected a cam. Am I missing something?

[harpwrench]

I didn't select a cam, then entered the 54 intake close for the 598 retarded 4 degrees instead. You'd leave that box at zero if just using a selected cam installed straight up. Another option is the S&S 625, but the exhaust opens a lot earlier which might kill the torque. Depends on the head porting and pipe

[tdrglide]

I would be looking at sending cylinders out to check fit for new flat top CP pistons, Manufacturers are pretty consistent and tight with their own piston tolerances. Worst-case scenario you have to find some take off cylinders and bore. I was able to swap pistons before with no issues.
0.030 mls head gaskets. Leave heads as is. It's a carb bike isn't it.

[billbuilds]

I would be looking at sending cylinders out to check fit for new flat top CP pistons, Manufacturers are pretty consistent and tight with their own piston tolerances. Worst-case scenario you have to find some take off cylinders and bore. I was able to swap pistons before with no issues.
0.030 mls head gaskets. Leave heads as is. It's a carb bike isn't it.

    Yes, it's a carbed bike and I was thinking along the lines of what you suggested, having really wanted to go that route from the git-go but...

[billbuilds]

I didn't select a cam, then entered the 54 intake close for the 598 retarded 4 degrees instead. You'd leave that box at zero if just using a selected cam installed straight up. Another option is the S&S 625, but the exhaust opens a lot earlier which might kill the torque. Depends on the head porting and pipe

     OK, I see now. Thanks

     The heads have been massaged a bit with slightly larger intakes but I'm not sure that they're set up for the S&S 625's. I've been using the stock touring header pipes with the internal protruding portion removed and the older off road-only SE slip-ons.

[Ohio HD]

I would be looking at sending cylinders out to check fit for new flat top CP pistons, Manufacturers are pretty consistent and tight with their own piston tolerances. Worst-case scenario you have to find some take off cylinders and bore. I was able to swap pistons before with no issues.
0.030 mls head gaskets. Leave heads as is. It's a carb bike isn't it.

    Yes, it's a carbed bike and I was thinking along the lines of what you suggested, having really wanted to go that route from the git-go but...

Bill I think this is also the most reasonable method to resolve the problem. You can resell the pistons you have and recover almost 100% of the new pistons cost.

Staying with a CP piston, I would not worry that the pistons finished size would change. I've used a lot of CP pistons over the years. I measure them all, and they measure what the spec sheet says they are.

CP-Carrillo BHM98FT is a flat top pistons, use a piston cc of -1.6 and you'll see about 9.1 corrected compression. Roughly 184ccp.

Mechanical compression with the heads and 0.030" head gasket is 10.10:1.

 

[Don D]

Drag Specialties has a nice flat top JE piston, 0910-5936 3.937" nominal

[Ohio HD]

On the bottom of the piston there should be two numbers; the CP job number and the piston part number (assuming its a standard CP shelf piston). The CP 98" pistons have the following designations:

BHM98FT -1.6cc 9.5:1 compression
BHM98-3 +3cc 10.0:1 compression
BHM98-6 +6cc 10.25:1 compression
BHM98-10 +10cc 10.75:1 compression

Bill, this is the piston data that Jamie posted in the Question on CP 98" Pistons thread.

If it were me, I'd source the "BHM98FT -1.6cc 9.5:1 compression" and not look back. They'll measure the same as the pistons you have regarding the fit into the cylinder bore.

[billbuilds]

    Thanks Brian. It's sure reassuring to know that I can count on a good fit with CP's.     Bill

[billbuilds]

Drag Specialties has a nice flat top JE piston, 0910-5936 3.937" nominal

      Thanks. I tried to find some info on fit and did not find JE's website very user friendly. Why it won't let you search by 3.937 bore size eludes me.

[Jamie Long]

Each of the CP BHM-98 pistons use the same forging, and there is more than sufficient dome thickness to machine yours as necessary. If you determine your chamber volume and what you need for a piston displacement to help you out of this jam i'll machine them for you no charge. We have a dedicated machine & piston fixture and I can turn them around in a day, just pay the shipping back. Just send an email to jamie@fuelmotousa.com and ill send you an authorization code

[Ohio HD]

That's a generous offer.   

[kd]

    

[billbuilds]

Each of the CP BHM-98 pistons use the same forging, and there is more than sufficient dome thickness to machine yours as necessary. If you determine your chamber volume and what you need for a piston displacement to help you out of this jam i'll machine them for you no charge. We have a dedicated machine & piston fixture and I can turn them around in a day, just pay the shipping back. Just send an email to jamie@fuelmotousa.com and ill send you an authorization code

    Wow, Jamie, that is a indeed a very generous offer.    I think that I'll cc those head again twice to firm up the chamber volume number now that I have a better grip on how to do it. Also need to carefully remove those rings. I'll send you an email later today.
                            Thank you  very much, Bill

[Buglet]

   Jamie is the man, I guess CP look for the easy why out and make money. As far as the deck thickness goes it's easy to check.   

[Don D]

It is a simple rule, just don't cut the dome leaving less than .200 cross section. Weight and skirt changes are factors as well in extreme cases where high domes are flattened. I am not suggesting this is the case here and I agree this is a generous offer.

[Buglet]

  I agree on the weight but unless you redoing the flywheels them have it balance to those pistons then I would be concern about it. When you going big bore not many are getting there wheels rebalance. The other thing is he will be taking weight off and being there bigger piston it should get him closer to the original weight. The only real way to check it out is to check the weight on both pistons after taking the dome off. 

[Don D]

As the pistons grow in weight the pins are reduced, with a mind on allowable minimums for reliability. It is an attempt to keep the performance domed pistons as close to stock as possible unless you just have a very different configuration such as a 20 degree dome. Those are just way heavy up top. This is how builders can use S&S wheels by using their target weight stated. Correction factors go up and down according to loss or gain of weight and this is not make or break stuff unless you again go very much out of spec.

[billbuilds]

    I CC-d both heads twice once again for peace of mind and I get 79cc pretty much dead-on for the front head and about a half cc bigger for the rear head. According to the spec sheet these pistons presently weight 421 grams. The pin diameter is .927. I looked into Kramm-Loxx (great idea Brian) and this appears to be a very popular size, at least for CP. Hoping that lightening the pistos some may help a bit balance-wise but only time will tell.
                                                          Thanks, Bill

[Don D]

Bill
I think you missed my point a little. I don't know about your crank but when a crank is balanced by the shops they ask for a gross weight. Rings, pins, clips included. If you drop 27g per piston, a reasonable swag for -10cc, that is X2. That changes the balance factor. Early TC cranks were balanced (factory) for ~1200g later cranks, cold forged ~1050. When the cranks are balanced for a certain weight that changes things, you give the shop the gross weight, they decide the balance factor and start drilling or filling. Piston weights are juggled some by changing the pins' weight, in other words the ID, within reason. None of this is news to Fuel Moto, they get it.
This build seems to be handled so I will drop off.

[billbuilds]

     HD Street Performance,

     The crank is the stock 1999 first year Twin Cam crank with the factory balance. I undestand what you mean now about increasing pin weight to make up for lost piston weight, thanks for pointing that out. Bill   
     

[Buglet]

    Bill  Did you measure the weight of the stock piston the came out of the motor.

[billbuilds]

Bill  Did you measure the weight of the stock piston the came out of the motor.

     I thought about that, Buglet. I know that I still have them, just have to put my hands on them and slide the rings off. I bet I'll have to go after some button batteries too. Thanks for the reminder.